Have you ever heard of Reso? Covering an area of ??12 square kilometres, this underground city running beneath Montreal, Canada is made up of a connected network of hotels, shopping malls, museums, office spaces and even a sports arena. hockey… With up to half a million visitors a day, this somewhat unusual urban project, born in the 1960s, has become a must. In addition to being a tourist attraction, this underground complex, the largest in the world, also serves as a refuge on freezing winter days.
Could such spaces be a temporary or permanent habitat for human beings? To shelter from extreme weather events due to climate change in particularly affected areas? Or, what if we were to colonize Mars one day, to protect ourselves from radiation and temperatures over 100°C? Technically, maybe…
But are we ready for a life without greenery or natural light, where freedom of movement is quite relative? Biochemist and science fiction writer Isaac Asimov imagined such cities where our species would remain cloistered, far from a perceived hostile outside… How compatible is human physiology with life underground? A fortiori if this one, less artificialized, were dark and humid? Could our body even survive it?
The idea of ??living underground for days or weeks is not new. For centuries, the city of Derinkuyu, more than 2,500 years old, sporadically sheltered up to 20,000 people 85 meters below the rocky surface of Cappadocia, in present-day Turkey, for protection from the weather and of the war.
However, it was only a little later that scientists became interested in the consequences of such a place of life on our species… In this case during the race for the Moon, during the Cold War. The great world powers then looked into the question to understand how the human body accommodated life in space!
To a large extent indeed, a cave presents living conditions comparable to those in space. Because, as in space or on Mars, the rhythm of day and night is different from that on Earth. In addition, the dimensions of the human habitat will be just as narrow as a cave. Others have explored the subject, literally, and more personally. A few months ago, 50-year-old Beatriz Flamini from Spain set the world record by living 70 meters below the surface for 500 days.
Perhaps the most obvious physiological change observed after a long period underground is the disruption of the sleep-wake rhythm, as evidenced by the testimonies of many participants in studies of this kind.
After a month without sunlight, and sometimes even despite the use of artificial lighting, the days begin to blur: when asked to note when they think a day has passed, they are actually rather on a two-day basis – with 34 hours spent awake and 14 hours asleep.
It’s as if they were all caught up in an endless jet lag. But the consequences are wider since there are still reports of lower job performance, hallucinations and poorer reaction time.
Where do these disturbances come from? Life is, in fact, a matter of rhythms, whatever the species considered – or very nearly so. They create predictability, which makes it possible to thrive in a stable and easy to anticipate world. Think of the life cycles of trees or furry animals that hibernate, adjusting to the changing seasons. Any disruption to this natural clockwork can jeopardize the survival of a species if it cannot adapt to it – climate change is a terrible and excellent example of this.
The human body is no exception to this rule as many of its vital functions follow a 24-hour cycle in line with the day/night alternation – which results from the rotation of the Earth. These are the circadian rhythms, of circa (almost) and dia (day).
It is assumed that these fluctuations are related to our metabolic activity: higher temperatures during the day increase our metabolism to support physical activity and lower ones are more relevant at night to decrease our energy consumption and promote sleep.
The sleep-wake cycle is the daily circadian rhythm with which we are most familiar. And as with all animals, it is more or less regular. It is governed by a central clock located in our brain. It is, more precisely, a network of about 20,000 nerve cells located at its base, in the hypothalamus. Contrary to what one might think, circadian rhythms persist even in the absence of any natural light.
And another surprise awaits us. Experiments in animals and humans deprived of light for several days have shown that the sleep-wake cycle actually lasts not 24, but 25 hours. After some time in the dark, the day-night and sleep-wake cycles will therefore become out of sync.
You’ve had a similar experience if you’ve flown, say across the Atlantic, and felt the effects of jet lag – which usually affects mood and attention. Smartphones and light pollution also interfere with our circadian rhythms, as they can act as “zeitgebers”.
Animal studies and epidemiological data have shown that persistent disruption of biorhythms is associated with a higher likelihood of developing chronic diseases later in life, such as diabetes and depression. So far, however, no experiments have assessed the long-term risks of prolonged underground life.
But life underground has other consequences. Along with disruptions in biorhythms, scientists have seen muscle damage, early stress response, and increased inflammation. This means that our body is in a state of hypervigilance due to the sub-optimal environmental conditions. It is a sort of flight or fight reaction that he is preparing to survive.
We can cope with it for a while through increased secretion of the stress hormone cortisol and a temporary increase in metabolism… But, over the long term, high levels of stress deplete the body’s reserves. and increases susceptibility to disease and infection.
And there is another reason why we need the Sun, its UV rays in this case: to generate vitamin D, which is essential for the proper absorption of calcium responsible for strong and healthy bones. . Years underground would thus increase the risk of osteoporosis (bone fragility). Our diet should compensate and provide the necessary vitamin D. This is what the 57 members of a sect who lived in an underground bunker without natural light in the Republic of Tatarstan did.
Despite these few experimental data, we still do not know, in detail, how life below the Earth’s surface would affect us over long periods of time. That’s why NASA is currently looking for four volunteers to live for a year in a 160m2 3D-printed environment, similar to the one planned for Mars, to learn more.
But the main challenge may well be mental, not physiological. As impressive as the performance of Spaniard Beatriz Flamini, who flew like a flower through her 500 days in the depths, she could leave her cave in an emergency. This will be impossible on Mars…or if we were to shelter from deadly conditions for years.
Human life has adapted over millions of years to survive in the small area between the ground and the air. It is therefore unlikely that our physiology and mind will instantly adapt to such unnatural conditions…
