Surviving the Next Doomsday Asteroid Impact

One day, as has happened many times before, a large asteroid thousands of metres in diameter will hit our planet.

The result of the impact will rain hot debris across the entire Earth, heating the atmosphere to an oven-like temperature and shrouding it in dust clouds. Fires will burn for years adding billions of tonnes of soot to the atmosphere, and then the global temperature will fall dramatically. Photosynthesis will grind to a halt. Almost all species of plants and animals will perish. The only survivors will be those lifeforms that are able to eat the remains of long dead life, such a cockroaches and deep sea creatures.

The collision of a large asteroid thousands of metres in diameter with our planet will almost certainly result in the extinction of most lifeforms. It is unlikely that humans would survive.

If such an impact happened now humans would not survive. The extinction of our species would be certain. Essentially, life on Earth would be reset to the point just after the last massive impact event 66 million years ago (the Cretaceous-Paleogene extinction event that caused of the extinction of the dinosaurs).

By the next century we are likely to have permanently inhabited colonies on the Moon, Mars and possibly even beyond, but there will be a few hundred inhabitants at best. And most crucially those colonies will not be self-sufficient. If Earth is rendered uninhabitable and our civilisation destroyed then those colonies will die soon after. It will be a couple of centuries until self-sufficient colonies with tens of thousands (preferably millions) of people exist away from our planet. Until then we will need sanctuaries on Earth where a significant human population can survive should a massive asteroid impact occur. And by 'significant' I mean several million people at least.

The cost of building such underground sanctuaries would be extreme, and it would take decades or longer before they were ready for habitation. Hundreds of billions of dollars would be needed each year, and a workforce of millions. But it is actually affordable and achievable, if only the world's governments could be less paranoid and divert some of their defence spending to the project. The total spending each year by NATO members (29 European and North American countries) is more than one trillion dollars. If just a quarter of that budget could be redirected then the underground sanctuaries for North America and Europe could begin construction. If the likes of Russia, China, Japan and their neighbours did the same then sanctuaries in their region of the world could begin construction, too.

After a century of construction and expansion, the main chamber of a vast underground sanctuary, deep beneath mountains in Europe, is home to a thriving city of a million people. Powered by geothermal energy, the sanctuary is a self-sufficient haven for humans, one of several spread around the Earth. Even the impact of a large asteroid, such as the one that resulted in the extinction of the dinosaurs and many other species, would not lead to human extinction if such facilities exist.

Each sanctuary would need to be hundreds of metres underground, with locations beneath mountains the most preferable. As well as hundreds of kilometres of tunnels and smaller chambers there would need to be huge areas for agriculture, and even larger caverns to create the feeling of space and distance we have evolved to need, if only to maintain the sanity of individuals. Power could be generated using geothermal technology.  Energy generation using geothermal power stations currently provides 30 percent of the electricity requirements of Iceland, with the Philippines not far behind that. It is a relatively clean and renewable source of energy and would be ideal for subterranean habitats. New power generation technology, such as nuclear fusion, will hopefully be perfected over the next decades, which could provide almost unlimited power capabilities.

One of the many farming chambers that surrounds the main city of each of the underground sanctuaries. Agricultural facilities such as this, powered by geothermal energy, or even fusion energy, provide food for millions for thousands of years in the event of a catastrophic incident up on the Earth's surface.

Once created the sanctuaries would need to be permanently occupied to a high capacity: at least 50 percent. This is necessary to ensure that the sanctuaries are fully functional and under constant maintenance. They should be regularly assessed to identify areas of improvement. They would need to be totally self-sufficient if the need for them arises. There would hopefully be many months, if not years, of notice before a large impact would occur, giving plenty of time for people to migrate into the sanctuaries.

There are ethical and moral issues with regards to who would be selected to migrate into the sanctuaries should an extinction level impact be confirmed. The choice of certain sections of the population is obvious: there needs to be specialists in all areas of science and technology (engineering, medical, computing, utilities, agriculture etc) and also educators; it will be essential that education levels are maintained for the generations that will live in the sanctuaries, so schools and universities would be required.  The majority of inhabitants, however, will be ordinary citizens with ordinary levels of education and skills. Whatever method is used to choose which of those individuals and families are chosen - a lottery, or genetics (to maintain the diversity of the gene pool) - there would be objections, protests and even wars fought no matter who is chosen (there would be nations of the world without any access to sanctuaries of their own). Great care would need to be taken to ensure that the security of the sanctuaries is maintained and the chance of sabotage is kept to a minimum.

The selection of who is to migrate to the sanctuaries, and how to prevent attacks on them as they travel there, will perhaps be a more troubled process than the construction of the actual sanctuaries themselves.

Of course, it would be best if a cataclysmic asteroid collision was prevented altogether. There are numerous proposals to deflect or destroy asteroids that are identified to be on an eventual collision course with Earth. None of them has yet been tried, and all of them required many years, or even decades, of warning. Research into such methods should be intensified and an effective asteroid defence system should be implemented as soon as possible. The underground sanctuaries would allow a small percentage of our human population to survive the aftermath, but they should be considered a last resort. The need for then should ideally never arise.

Developing the ability to deflect or destroy asteroids on a collision course with Earth is essential to prevent the extinction of our species. A joint NASA and ESA mission in 2020-2021 will perform the first test of such technology.

If we construct several underground sanctuaries around the world, and deploy a system capable of destroying or deflecting asteroids, we stand an excellent chance of surviving as a species even if an object many tens of kilometres wide collides with our planet, and of eventually repopulating the surface once its ecosystem has recovered.

But, before the creation of the underground sanctuaries can happen we need to cultivate a will to change our thinking and work together for the greater good and the very preservation of our species and the wider ecosystem on which we depend. If we fail to do so we will fail all of humanity, and our extinction will be assured.

Preserving Earth Life

Opened in 2008, the Svalbard Global Seed Vault, located on the Norwegian island of Spitsbergen far north of the Arctic Circle, is the world's largest store of seeds. It was built as a way to preserve seeds from global catastrophes, and is a backup for the hundreds of other seed vaults around the world, some of which have already been lost due to war and natural disasters.

The modest entrance to the Svalbard Global Seed Vault on the Norwegian island of Spitsbergen. The actual vaults are deep within the mountain. The vault holds close to a million samples of seeds.

The seeds are stored in vaults deep in the permafrost, which helps to keep the seeds cold enough for preservation. But despite being built to protect seeds from potential disasters such as climate change, climate change is already threatening the vault itself. The Norwegian government is having to make modifications just a decade after it was built. This is an ominous sign that whatever we do to preserve life and all that we have achieved on our planet (the Arctic World Archive opened nearby in 2017 as a data and knowledge store) it is going to be a losing battle.

We need to think bigger if we are to protect life and our knowledge and achievements from oblivion.

The colder the better is the key to very long term preservation, and natural environments that offer such low temperatures for 'free' are highly abundant within our Solar-System.

Inner Solar-System Vaults

The Moon has areas of permanent shadow at its poles, and a NASA instrument on board India's Chandrayaan-1 probe confirmed the presence of water ice in those regions. Those areas, as well as being ideal locations for human colonies, would make excellent locations for preservation vaults. The close proximity to Earth would make the vaults relatively easy to access if/when Earth's climate causes major problems that require access.

A vault so close to Earth would also be vital if our current civilisation is wiped out by a catastrophic event, such as a nuclear war or an asteroid impact. It could take centuries, even millennia, for a new civilisation to advance to the point where space travel is possible.

On Mercury, the closest planet to the sun, there are also areas on the planet's surface that are in permanent shadow in craters at its north pole. The Messenger probe that orbited Mercury between 2011 and 2015 discovered billions of tonnes of water ice in those areas. It proves that preservation vaults can be built on bodies very close to a star, providing there is no atmosphere to distribute heat to those areas of permanent shadow.

The planet Mercury. Despite being so close to the sun the planet still has a large amount of water ice on its surface. The ice is located in craters in areas of permanent shade. It would be possible to construct preservation vaults that would provide protection for life and knowledge for millions of years.

A vault on Mercury would only prove sensible if sizeable human colonies were to be established there, which is unlikely. Venturing so close to the sun is not something that I predict will happen for humans. Perhaps colonies in the clouds of Venus will be as as near as we will get.

Human colonisation of Mars is almost certain to happen this century. There is abundant ice on Mars, which will be heavily exploited by the colonists to provide water, oxygen and fuel. Those colonists should create preservation vaults as a matter of urgency. It should be one of their top priorities once the essential activities for their survival are established and running smoothly. Areas within the ice mines could be designated for such a purpose.

Outer Solar-System Vaults

There are many large bodies in the Kuiper Belt, such as Eris, Haumea and Makemake, that would be worthy locations for preservation vaults, and there will be hundreds more waiting to be discovered. Those larger bodies are more useful that the millions of small bodies out there, if only because they provide at least a useful level of gravity, even if it is only around 5 percent that of Earth.

The best known example of a large object in that region is Pluto. The mountains and crevasses of that dwarf planet, which can rise over six kilometres in height, are examples of places in the outer Solar-System that would be ideal locations for such vaults.

The surface of the dwarf planet Pluto. The Al-Idrisi mountains shown consist almost entirely of water ice, and the plains are mostly nitrogen ice. With a mean surface temperature of -229 Celcius it would be an ideal place for preserving life and knowledge for many hundreds of millions of years at least.

Even on the surface of Pluto the temperature is low enough to provide an excellent level of preservation, but vaults should still be constructed deep into the ice to protect against meteorites, and eventually from theft. Building inside mountains means access to the vaults can be horizontal, which would ease construction. Several preservation vaults should be constructed, each on a different dwarf planet and on radically different orbits.

Interstellar Vaults

Ultimately the sun will flare up and expand, consuming the inner Solar-System vaults. The outer Solar-System will suffer too, with the extra heat threatening the stability of vaults in that region. A truly long-lasting vault, that could survive for billions of years, should not be in the Solar-System at all.

The interstellar void between stars is the coldest and most benign area in the galaxy. It is not completely empty, with rogue planets and smaller bodies drifting for aeons out there in the frigid darkness. Finding one is not easy, but such a world is an ideal place to build a safe and secure preservation vault (and surprisingly, it would also be suitable for an interstellar human colony).

Instead of hunting for interstellar objects on which to construct the vaults it would be easier to create our own. By redirecting some of the smaller Oort cloud objects away from the Solar-System (or even constructing our own purpose-built station for the vault) we would have easily available bodies on which to preserve whatever we wish. Ideally there should be human colonies nearby.

Although in the Kuiper Belt, Arrokoth (pictured here) is a good example of the kind of small object we can find in the Oort Cloud. At just under 20 kilometres in diameter, a body like this could host a preservation vault and be redirected onto an interstellar course to hide between the stars.

It may sound almost impossible, and undesirable, for a human colony to be located away from the obvious benefits of a star, but it is not so. My 2018 article 'Secret Colonies Between the Stars' explains why not, and gives reasons why such colonies would actually be an essential part of our colonisation efforts, and even on which the ultimate survival of our species could well depend.

Where ever we humans go, from the inner and outer Solar-System to interstellar space and other star-systems, we should construct life and knowledge vaults to preserve as much of what has been achieved on Earth as possible. It should be an essential part of colony building wherever that may be. In billions of years time it will then be possible for our very distant descendants to benefit from our endeavours, even if it is just to know where all that they take for granted actually comes from.

Hopefully they'll be able to enjoy bread just as we do today.

Intelligent Life on Earth 56 Million Years Ago

Almost 56 million years ago the Earth's climate suffered a fast and significant warming. It's quite possible that an ancient industrial civilisation was intensely active at that time.

The warming, known as the Paleocene-Eocene Thermal Maximum, was due primarily to a rapid rise in carbon dioxide levels in the atmosphere. The event may have happened over a very short period of time, possibly as short as 1,500 years, and the number of species that went extinct rose to several times that of normal levels.

During that period the average temperature was already several degrees higher than today, meaning there were no polar icecaps, but nevertheless sea levels rose due to thermal expansion. If a civilisation existed at that time it would have had to deal with relatively rapid changes to its coastal habitats and to weather patterns.

The parallels with what is happening to our climate right now are eerily obvious.

If the warming was due to the activities of an advanced civilisation it is likely that the rapid climate change was responsible for its demise. Finding out exactly what happened could give us valuable information that may help save our own civilisation.

A city on Earth, more than 56 million years ago. During that time the climate warmed rapidly. The civilisation that existed at that time would have found its existence increasingly threatened as a result of its unsustainable use of the planet's resources. We are facing the same threats. We must find a way to avoid making the same mistakes that that were made by previous advanced civilisations on our planet.

Finding the evidence of a civilisation that existed 56 million years ago is extremely difficult. Over such a timescale geological processes (plate tectonics, weathering and erosion) would wipe the Earth's surface clean of almost all traces of buildings, technology and transport infrastructure. For example, if our civilisation ended now, within a few million years there would be nothing left of any artefacts we created, or of our urbanisation which only covers about one percent of the surface of the planet. The only signs would be spikes of metals and possible traces of plastics, and certain radioactive isotopes in sedimentary deposits (if we destroy ourselves with a global nuclear war the evidence would be readily detectable millions of years in the future). Such chemical signatures would be the only evidence that we could reasonably hope to find, and even these would be very hard to find if the civilisation was only industrially active for a few hundred years.

If the civilisation had developed space flight capability then that would offer more hope. The best place to find evidence of its existence would be away from Earth. There may well be ancient space probes orbiting within our Solar-System, and on the surface of its airless worlds, that have been preserved quite well. Of course, the vastness of space would mean finding them will still be a challenge, but as we map the surfaces of many of the Solar-System's planets and moons in ever greater detail we may soon find something.

A previous industrial civilisation on Earth millions of years ago may have developed spaceflight capabilities. Evidence of their activities away from our planet would be the best way to prove their existence. Spaceships such as this one may have been built to enable the colonisation of our Solar-System and beyond as an attempt to survive the warming event that their activities on Earth inadvertently caused.

I discussed the possibility of finding evidence of ancient mining activity in the asteroid belt in my earlier article titled 'Pre-Human Technology in the Asteroid Belt'. If such evidence is found then it is possible that the civilisation felt the need to construct interstellar ships to colonise planets around other stars, probably as a way to preserve at least some of their knowledge and culture as the Earth rapidly became hostile to them. Right now there could be one or more Earth-like planets in nearby star systems that are thriving with the descendants of Earth life. The civilisation that populated those worlds may still exist, or another may have since risen to take its place. Perhaps they even returned millions of years later and influenced the rise of human civilisation on Earth.

Colonists from a pre-human Earth civilisation walk upon their new world. Their civilisation's activities on Earth millions of years ago damaged the climate to such an extent that their species faced extinction. Colonising an Earth-like planet orbiting another star seemed to be the only way for their species to survive. Perhaps one day, when we are able to visit other star-systems, we will discover life descended from Earth life.

The rapid climate change 56 million years ago is not the only time a possible industrial civilisation existed before our current one. There has been speculation that there could well have been a dinosaur civilisation, possibly created by a descendent of a Troodon, that would certainly have ended when the Cretaceous-Palegene extinction event occurred 66 million years ago. And extinction events such as the Toarcian turnover 183 million years ago could also be related to yet another industrial civilisation abusing the Earth's resources.

Discovering proof of the existence of pre-human industrial civilisations on Earth is quite an intoxicating prospect. Life has existed on our planet for at least 3.5 billion years, and life with very complex brains, and consciousness, has existed for hundreds of million of years. There has been ample time for multiple civilisations to rise and fall. The chances are that at least one of them would have developed to an industrial and even highly technological level.

With the right kind of research we will be able to find strong evidence of the existence of such a civilisation. Paleontological and geological expeditions with the sole purpose of uncovering that evidence should begin at the earliest opportunity, and all space missions, past and future, should have their data examined to see if evidence of an ancient technological civilisation is present in our Solar-System. It is exciting to think that there may well be evidence hidden within the vast amount of data already collected by NASA and other space agencies.

If evidence of an ancient pre-human industrial civilisation on Earth is discovered it would have profound implications. It would mean that intelligence, at least to our own level, is a natural and relatively common evolutionary occurrence. And it would mean that industrial civilisations on other life-bearing planets should be common, too.

Climate Change - The Point of No Return

We are on the verge of rendering our planet uninhabitable, unless drastic action is taken.

Scientists have recently announced, quite alarmingly, that climate change on our planet could soon reach its tipping point. We are only decades away from that moment. It will happen within a generation.

Lowering our carbon dioxide emissions is no longer enough. Carbon dioxide needs to be extracted from the atmosphere. And we need to start doing that now on a large scale. Otherwise, global warming will enter an unstoppable feedback stage, where warming triggers more warming, which triggers even more warming. Such feedbacks include the release of methane due to the thawing of permafrost, loss of snow cover, the melting of Arctic ice, warming seas, and the loss of forests. All of those will reduce our planet's ability to reflect heat and absorb carbon. The number and intensity of forest fires will increase dramatically, releasing huge amounts of greenhouse gases into the atmosphere. This will only magnify the feedback effect even more.

Global warming will result in an increase in the frequency and ferocity of forest fires. Such fires add vast amounts of greenhouse gases to the atmosphere.

The end result of such warming is not clear, but at best it will significantly limit the areas of the Earth where humans can comfortably survive. At worst it will render the whole planet uninhabitable, water will boil away, and the Earth will suffer the same fate as Venus,

Even the 'best case' scenario will cause massive migrations, and subsequently major wars over dwindling food, water and energy resources. Such wars will only add to the warming feedback. As the desperation of governments increases those with nuclear weapons at their dosposal will eventually use them. Countries such as Israel, India, Pakistan and Iran - all nuclear powers and all located in what will become some of the worst affected regions - will feel compelled to use such weapons as countries to their north attempt to stop the migration of their populations. This will cause tremendous damage to the environment, and render even more areas uninhabitable. And there would be a very high chance that a limited nuclear war would escalate rapidly into a global nuclear conflict.

At that point our current technological civilisation will end, and with it humanity's chances of spreading beyond the Earth to become an interplanetary, and then an interstellar, civilisation. Our ability to preserve our species will have slipped away.

If he climate change wars turn into nuclear conficts they will bring to an end our current technological civilisation. Humans may have a change to develop another advanced civilisation one day, but it is likely to take centuries or even millenia to get back to anything like that which we have achieved today. Without forewarning such a civisation is almost certainly doomed to suffer the same fate as the one before it.

Such a dramatic warming of the atmosphere will cause a rapid rise in sea level, raising it to well over a hundred metres higher than it is at present. In addition to the billions who had already perished in the wars of the previous decades, hundreds of millions more will die. Survivors, who will eventually number just a few tens of million at best, will struggle to live in the few remaining higher altitude lands. Their existence will be basic and medieval. It will be a return to the dark ages. Thousands of years of progress and knowledge will have been lost.

Only then, with the destructive output of our current civilisation at an end, can the climate of Earth have a chance to stabilise. The ice caps will reform and the sea level will reduce. After many centuries flora and fauna will start to recover. Only at that point, humans, if they are not extinct, will have another chance at building a technologically advanced civilisation.

It's unlikely that we can prevent a climate change disaster on our planet, but we can improve on the 'best case' scenario if we start doing three things right now:

1. Preserve Knowledge of our Discoveries and Inventions

We must help survivors in the post-climate change world avoid repeating our calamitous mistakes, and allow them to 'fast-track' through what has taken our current civilisation thousands of years to learn, discover and invent.

A comprehensive record of our achievements (and of our destructive actions, so that mistakes are not repeated), must be preserved in a way that they can understand, and in a way that will last for thousands, even millions, of years. We must investigate how we can provide such knowledge to our distant descendants, and then store it in multiple safe yet easily accessible locations, including locations elsewhere in the Solar-System.

DNA could be the ideal solution to extremely long term information storage

Using paper or digital media to preserve knowledge will only be suitable for a few hundred years or so, but such means should be used initially, as our descendants will be able to understand and use these forms of storage.

But a more radical solution is needed for complex and advanced knowledge that will be useful once any new civilisation progresses to a certain level. Storing information in DNA is one such solution. It's been shown that if the DNA can be kept at sub-zero temperatures the information will maintain its integrity almost indefinitely. DNA information stores would be best located away from Earth, perhaps on the Moon in the permanent darkness of one of the polar craters, and deep within some of the planetary bodies of the outer Solar-System.

2. Work to Avoid or Delay the Climate Change Tipping Point

Even to just delay the tipping point we need to start work now.  The small steps some of us are currently taking are utterly inadequate.

Our civilisation needs to fast track the very widespread use of electric vehicles by banning petrol and diesel engines within a decade. All developed countries need to push this idea hard, and give incentives to developing countries to do the same. And governments need to increase vastly the funding for research into clean energy, and get nuclear fusion working. The generation of electricity by burning coal and oil has to stop on a worldwide scale without delay. Countries like China appear to be increasing the use of fossil fuels for energy generation, which is going to be catastrophic if it is not prevented.

But that alone will not be enough. We need to start undoing the damage we've already done.

Various climate engineering projects should be started immediately. These are possible with current technology. To help reduce global warming solar radiation management needs to be implemented to reduce the sunlight absorbed by the atmosphere. Relatively simple things can be done such as seeding clouds with sea water to brighten them (and therefore reflect more sunlight), and using pale roof colourings and promoting the expansion of polar ice.

Removing carbon dioxide from the atmosphere using facilities such as this is possible. It can be stored in hard pellets which can be buried deep underground.

We need to start removing some of the excess greenhouse gases that our activities have pumped into the atmosphere. This can be done directly using machinery that would then store the extracted carbon dioxide deep underground, and indirectly by promoting natural processes, such as extensive tree planting to reverse deforestation, and ocean fertilisation to add nutrients to the upper oceans to increase carbon dioxide absorption.

And the growth of our human population needs to be controlled, especially in developing countries. We can no longer support such large increases, which are generally located in regions that can least support it. It should be stabilised as soon as possible, and allowed to reduce naturally to a more sustainable level.

Doing all of the above is the best chance the Earth has of remaining a viable place for humans to live. But it is likely to only buy us some time - nothing more. We need to establish our civilisation elsewhere to properly secure our future.

3. Secure the Survival of Our Civilisation Independent of Earth

There's a significant possibility that the feedback warming will not stop, and if that is the case the Earth could indeed end up in a state similar to that of Venus.

The only way to ensure the continuation of our species beyond that event is to make sure that there are self-sustaining human colonies beyond Earth, on planetary bodies such as the Moon, Mars, and especially on what is looking like the best location: Titan, the largest moon of Saturn.

As well as colonies on planetary bodies there should be very large space habitats constructed throughout the Solar-System that would house tens of thousands. The resources to build such massive facilities can be found in the asteroid belt between Mars and Jupiter.

We will have to mine asteroids on a large scale to construct the off-world facilities humans will need to live independently from Earth

Extracting those resources would be relatively easy due to the very low gravity wells of the asteroids. The establishment of mining operations there must be an immediate priority. The space habitats could be constructed in the asteroid belt and then maneuvered into their final positions, either into planetary orbits, or into their own independent orbits around the sun.

In parallel to the development of colonies around the Solar-System there needs to be development of interstellar missions with the goal of establishing human colonies around other stars. More and more exo-planets with the potential for colonisation are being discovered all the time, with one, Proxima Centauri B, only 4.3 light years away.

The planet Proxima B, which orbits Proxima Centauri 4.3 light-years from Earth. It's the closest known Earth-like exo-planet, and has huge potential as a suitable destination for our first interstellar colonisation mission.

It would be a mammoth undertaking, and there are incredible engineering challenges to overcome, but investment in the rapid development of the methods and technology required is essential to build such habitats in time. As well as providing immediate funding, it should be the priority of governments to ensure that education systems are geared to maintain a constant supply of highly capable scientists and engineers to make it a success. The long term survival of our species depends on it.

If all three of the above steps - preservation of our knowledge, delaying or avoiding the tipping point, and establishing large self-sustaining human colonies elsewhere in the Solar-System and beyond - are pursued with the resilience, determination and creativity that our species has in abundance, then we will survive.

But we must start now.

Right now.

Sanctuary Entrance Found on Mars

It's quite possible that NASA has found the entrance to an underground sanctuary on Mars.

In my earlier article, 'Where Did All The Martians Go?', I discussed the dramatic change in the climate of Mars, and the need for as many of its inhabitants as possible to evacuate the planet. But most would have had to stay. I wrote about the huge underground shelters that would be required: long term havens that would enable millions to survive for many generations until they too could be evacuated, or until the planet's atmosphere and environment recovered.

The entrance to one of those havens may now have been discovered.

Near the south pole of Mars: the possible entrance to a huge underground survival facility, capable of sustaining thousands, if not millions, of Martians as their climate failed and surface survival became impossible.

Found near the south pole of Mars by NASA's Mars Reconnaissance Orbiter, the hole, hundreds of metres in diameter, is possibly one of the main entrances to a vast underground complex. It was probably used for vehicles, including aircraft and spacecraft.

And there are likely to be many more such entrances, large and small. One example can be found on the slopes of Pavonis Mons, an extinct shield volcano in the Tharsis region of Mars near the planet's equator. Shown below, the hole could have been intentionally created to allow Martians to utilise the lava tubes of the extinct volcano to access a network of underground sanctuaries. Its equatorial location would have been very useful for more economical launches of spacecraft.

Such holes can be naturally formed. This one is on Pavonis Mons, and is thought by some to be the collapsed roof of a lava tube. It could also have been intentionally constructed as an entrance to a massive shelter beneath the extinct volcano.

There has been no real evidence of unnatural surface activity on Mars, which suggests three possibilities: all of the inhabitants have now left, they are surviving solely by subterranean means, or they have all perished.

The first possibility is the most unlikely. The effort to evacuate millions of inhabitants from the surface of a planet would be a gargantuan undertaking for even the most advanced civilisation. While a huge number of Martians will indeed have left the planet, most would have had to remain. Even if evacuations continued for thousands of years, which is a possibility, the remaining population would have continued to renew as generation after generation were born. And most may have wished to remain, having physically and mentally adapted to their subterranean existence.

The second possibility of the inhabitants still surviving underground right now also seem very unlikely, at least in large numbers. There is no evidence of the emissions that would be observable if there was a massive population still living there, even underground. Of course, they could be withholding their emissions to prevent detection and living in a perfectly contained and self-sustaining biosphere, but this would be almost impossible to achieve to a level where no emissions at all were made.

A vast underground Martian city, home to possibly millions when war and climate change rendered the planet's surface uninhabitable. Could there still be millions of Martians living in such cities, undetectable within a perfectly contained biosphere?

Unfortunately the most likely possibility is that all the inhabitants left in the subterranean facilities eventually died. If the sanctuaries were designed well enough they could have survived for thousands of generations - a remarkable achievement. But many millions of years later the likelihood of the sanctuaries still functioning is minuscule. At best there may be a few hundred Martians still eking out an existence in the huge and now deserted underground cities. If so it must be a lonely and grim experience.

A cavernous space like this, clearly artificial, could be found beyond the sanctuary entrance. While the sanctuary is most likely uninhabited now, there could still be a small community of Martians living there. We need to take great care when we finally start to explore such places.

The discovery of the huge potential sanctuary entrance near the south pole of Mars reinforces the need for a substantial human colony to be established to enable its exploration. If there is indeed an abandoned network of vast underground cities on the planet we need to explore them as soon as possible.

The first colonists on Mars explore one of the entrances to an abandoned underground city

As well as the thrill of discovering the remains of a lost extraterrestrial civilisation (with the remote potential of encountering surviving members of that civilisation), we would have the opportunity to find out exactly what calamitous events happened millions of years ago which forced the planet's population to retreat underground.

Thrilling indeed...