The idea of underground spaces as temporary or permanent habitats has already been considered to deal with extreme events or even to colonize Mars. Underground life experiments have even been carried out over several months, but could we live underground in a sustainable way ? Without natural light, without greenery and where the perception of time is lost, what would happen to our mental health and what repercussions on our body ?
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 this type of space constitute a temporary or permanent habitat for human beings ? To shelter from extreme weather events due to climate change in particularly affected areas ? Or, if we were to colonize Mars one day, to protect ourselves from radiation and temperatures over 212°F (100°C) ? Technically, maybe...
But are we ready for a life without greenery or natural light, and where freedom of movement is quite relative ? The biochemist and author of S.-F. Isaac Asimov had imagined such cities, where our species would remain cloistered, far from an outside perceived as hostile... If for our fictional descendants this environment has become the norm, it would put our minds of the 21st century to the test.
Physically, things would not be simpler. How compatible is human physiology with living underground ? A fortiori if this one, less artificialized, were dark and humid ? Could our body even survive it ?
Endless jet lag
Without going back to the oft-dismantled myth of the “Caveman”, 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, to protect themselves 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 Spaniard Beatriz Flamini 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 blend together: when asked to write down 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.
Corollary: this slowing down of time is also perceptible at the level of the counting of days. After spending 366 days in a cave near Pesaro in Italy in 1993, sociologist Maurizio Montalbini thought only 219 days had passed.
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 poorer performance at work, hallucinations and poorer reaction time.
The rhythms of life
Where do these disturbances come from? Life is, in fact, a matter of rhythms, whatever the species considered (or almost).
They create predictability, and predictability makes it possible to thrive in a stable and easily predictable world. Think of the life cycles of trees or furry animals that hibernate, adjusting to the changing seasons. Any disruption of this natural clockwork can jeopardize the survival of a species if it cannot adapt to it (climate change is an example as terrible as it is excellent).
The human body is no exception to the rule, since many of its vital functions follow a 24-hour cycle in phase with the day/night alternation (which results from the rotation of the Earth). These are the circadian rhythms, of circa (almost) and dia (day).
Take the case of our core temperature. According to the textbooks, it is 98.24°F (36.8°C). In fact, if we record it in several thousand people over the course of a day, we will see a sine wave appear - a curve that goes up and down : our body temperature is at its lowest in the morning, and reaches its peak late afternoon.
It is assumed that these fluctuations are related to our metabolic activity : higher temperatures during the day increase our metabolism in order to support physical activity, and low ones are more relevant at night to decrease our energy consumption and promote sleep.
The concept of “Zeitgeber”
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 around 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 carried out on animals and humans deprived of light for several days have shown that the sleep-wake cycle actually lasts not 24, but 25 hours (note the “circa” in circadian)... After a certain time in darkness, the day-night and sleep-wake cycles will therefore become out of sync.
Scientists say that the second is "free" in the absence of an external source of wedging, in this case the Sun. The latter is called “Zeitgeber”, or “time giver” in German. We need this Zeitgeber to regularly reset our sleep-wake cycle to stay in tune with the natural rhythm of day and night.
In a cave where the sun's rays do not penetrate, nothing more comes to align our biological rhythms with the environment, for lack of Zeitgeber. The perception of time is therefore lost...
You've had a similar experience if you've flown, for example across the Atlantic, and felt the effects of jet lag - which typically affects mood and attention. Smartphones and light pollution also interfere with our circadian rhythms, as they can play the role of Zeitgeber.
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.
Stress, vitamins… the other consequences of an underground life
But life underground has other consequences. Along with disruptions in biorhythms, scientists have found muscle damage, early stress response, and increased inflammation. This means that our body is in a state of hyper-vigilance 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 cortisol, the stress hormone, and a temporary increase in metabolism...
But over the long term, high levels of stress deplete the body's reserves and increase susceptibility to disease and infection. It is a common cause of depression and burnout for employees who have endured stressful conditions for years. Restricted and enclosed spaces elicit similar reactions. Astronaut Fred Haise had contracted an infection during the disastrous Apollo 13 flight, caused by Pseudomonas aeruginosa, a bacterium that usually only affects immunocompromised people.
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.
Children of the Sun...
Despite these few experimental data (which hardly make you want to...), we still do not know in detail how life under the surface of the Earth 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 160 m2 3D-printed environment, similar to the one planned for Mars, in order to learn more.
But the main challenge may well be mental, not physiological. As impressive as the performance of Beatriz Flamini, who flew like a flower through her 500 days in the depths, she could leave her cave in an emergency. It won't be possible 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 !
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