Of course, there are others who’d live underground just to escape their smartphones. “In a cave, you have this sensation of freedom,” says Christian Clot, an explorer, researcher, and founder of the Human Adaptation Institute, with which he ran the “Deep Time” experiment.

“We were 15, seven female, eight male,” Clot tells IFLScience. “We were absolutely shut off from any information from outside […] and for us, the experience was really, really nice and really creative, and everybody was really happy.”

If you venture underground or into a cave, however, your experience may differ. So, what’s life really like when you can’t see the Sun?

You won’t be the first

“[There were] 19 people before us” who had attempted life in a cave, Clot tells IFLScience. Some were researchers, such as Michel Siffre, the geologist who more or less kicked off the whole retiring-for-weeks-into-a-dark-cave thing back in the 1960s, or Maurizio Montalbini, a sociologist who kept returning to life underground despite by all accounts hating the experience – “are you trying to be funny?” he asked a reporter in 1998 who suggested he might prefer subterranean life. 

“I’m not going back in there. I need the Sun,” he said. “I used to dream about the dawn. It’s an experience I would not repeat.” (He would later repeat the experience in 2006 for a period of 235 days.)

Then there are the enthusiastic amateurs. There’s Stefania Follini, an Italian interior decorator who spent 130 days in a sealed cave with no human contact in 1989 at the behest of Montalbini and NASA scientists; and Beatriz Flamini, a Spanish extreme sportsperson who lived alone in a cave for 500 days for… well, just for the hell of it, it seems.

And all that is to say nothing of the many, many people who have lived underground, or in caves, or similarly ensconced from the outside world throughout history. There were the infamous “mole people” of New York – fewer now than there were in the 1980s, but the population of unhoused people living in the tunnels and infrastructure under the Big Apple still numbers in the hundreds, if not thousands. There was the city of Derinkuyu, in Turkey’s Cappadocia region – an underground city home to up to 20,000 people across 18 levels reaching progressively deeper into the Earth. And in modern-day Australia, there’s a whole town where half the population lives in underground caves, or “dugouts”, to escape the punishing heat.

But what would life underground actually be like? And what happens to the people who live it? 

Well, it depends.

It’s more psychological than physical

“Physiologically – if we talk about only physiology – nothing changed a lot,” Clot tells IFLScience. The cavers’ hearing got sharper, he says, as they adapted to a life where sight was no longer so much more important than the other senses; so too did their sense of touch, for the same reason.

That said, you certainly should expect some physical effects. Flamini reported feeling achy and stiff during her stay, losing both the will and the strength to exercise regularly in the cave; Follini lost her menstrual cycle. Siffre battled hypothermia: “my feet were always wet, and my body temperature got as low as 34°C (93°F),” he told Cabinet in 2008. 

We were really experiencing [life] out of time.

Christian Clot

And many of those who have ventured into the darkness have come out noticeably lighter. Follini reportedly lost about 9 kilograms (20 pounds) over her 130-day stay, coming out noticeably paler and thinner; Flamini lost 5.4 kilograms (12 pounds), with “the baby fat on her cheeks […] gone” when she re-entered the world, the New Yorker reported in 2024. Montalbini, in his third round underground, came out around 12.7 kilograms (28 pounds) lighter, despite taking with him a high-calorie diet inspired by those given to astronauts.

What’s the confounding variable there? It’s the result of a much weirder, much more impactful change to the human body when it’s taken from the sunlight: our sense of time goes completely haywire.

“The rhythms of everybody in the cave changed completely, without any similarity to outside,” Clot tells IFLScience. The group weren’t sleeping according to the Earth’s rotation; nor were they synced to each other, with some people sleeping for huge stints and others much less.

“We were really experiencing [life] out of time,” Clot says. “Out of society.”

It may be a strange outcome, but it certainly wasn’t unexpected. It happens to everyone, and it’s never the same: for Follini, her days stretched out to about 35 hours – 25 awake, 10 asleep – while Flamini occasionally went three days without sleeping. Siffre’s internal clock was only off by a tiny bit – “my sleep/wake cycle was not twenty-four hours, like people have on the surface on the Earth, but slightly longer – about twenty-four hours and thirty minutes,” he said in 2008 – but some of his research subjects had rhythms as long as 48 hours, effectively turning two days into one. “They would have thirty-six hours of continuous activity followed by twelve to fourteen hours of sleep,” Siffre explained. 

“It became common,” he said. “All of the other people I had put underground caught a forty-eight-hour sleep/wake cycle, except for me.”

With these stretched-out lives, eating gets more sparse – so you lose weight. You also lose days, in a way: almost everyone who lives away from the sunlight for any extended period of time comes out thinking they’re weeks or even months away from where the rest of the world is at. 

“I descended into the cave on July 16 and was planning [to] finish the experiment on September 14. When my surface team notified me that the day had finally arrived, I thought that it was only August 20,” Siffre recalled. “I believed I still had another month to spend in the cave. My psychological time had compressed by a factor of two.”

Other cavers reported the same. Asked what she thought the date was a few weeks before her stay ended, Follini guessed March 7 – in fact, it was May 4 – and Montalbini similarly underestimated his elapsed time underground by more than 150 days. Flamini, when told it was time to leave her cave, was shocked, saying that the whole thing had felt like “just a moment – a single night.”

“In the cave, the line of time disappears, and everything floats around you,” Flamini told The New Yorker. “There is no past, there is no future. Everything is present, everything is a while ago, and it’s all brutal and strange.”

You might just like it – or you might really hate it

If the main changes weren’t physiological, then what would Clot say was the most noticeable effect of cave living? It’s probably not what you’d expect: “the main one was an increase in creativity,” he tells IFLScience. “People were more creative in the cave.”

With physical and information freedom stripped away, the cavers found their mental worlds opening up. “They felt more free,” Clot says; “they felt like they were more able to have a control of their own life. And because you have more control, you feel a bit more confident.”

You have time to think, you have time to consider a situation, you have time to discuss with people. And because of all that, the result is more creativity.

Christian Clot

Free from the constant barrage of social media notifications, the participants’ brains had time to chill out and do what they do best: “with Tiktok, Netflix, Instagram, email, sounds, noise […] at each hour of your life – that is tiring for the brain,” Clot tells IFLScience. “Your brain is always working to try to understand the new information coming, and you can’t have space for other things.” 

Without all that, though, “you suddenly have time,” he says. “You have time to think, you have time to consider a situation, you have time to discuss with people. And because of all that, the result is more creativity.”

Of course, it helped that Clot had chosen his cave so particularly. It “was a really huge one, more than 5 kilometers [3 miles], with a lot of different rooms with different levels, so we’re able to discover, to explore, to see very nice things,” he recalls. “It was like being outside, but underground. It was quite perfect.”

Clot’s team tracked the participants’ emotions via regular questionnaires, plus basic physiological data from sweat and heart rate sensors. For previous cavers, however, their mental well-being was recorded with video feeds and diaries, and with startlingly different results from those witnessed by Clot and his team: the footage often showed what could reasonably be described as a descent into insanity, with Flamini experiencing hallucinations and outbursts of anger, and Follini creating cardboard friends to converse with – along with “a beer keg, several bottles of wine, a loaf of bread, a chess set, a top hat and a cat,” papers reported at the time. Siffre suffered depression after his stays; more than one attempt has resulted in suicide.

“There was no difference between what I was feeling then,” Flamini said of her time in the cave, “and what I understand as death.”

What could account for such radically opposite experiences? For Clot, the answer was obvious: everyone else had gone in alone. 

“Alone, in a cave, you don’t have anything,” Clot tells IFLScience. “You have just dark walls, and humidity, and it’s cold, and you don’t have things to [let you] just get out of your own brain. And if you don’t have other people to support you when you have some difficulties or tiredness or whatever, it’s really hard.” 

I’m really confident to say that a solo expedition in the cave will never end in a good way.

Christian Clot

Almost everyone who had gone underground before Clot and his team was either alone, stuck in small, isolated areas, there against their will, or some combination of the three, Clot explains – and that was a recipe for disaster. Humans – even the most extreme introverts among us – are a social species, and with no light, no time, and no human interaction, the researchers “experienced some mental trouble, some depression,” he explains, both in and out of the caves. “All of them experienced huge difficulties,” he says, “months after they come back into their normal situation.”

“I’m really confident to say that a solo expedition in the cave will never end in a good way,” he cautions. “I don’t have any example from the 19 people who did that before ending in the right way.”

Recovery might take longer than you expect

So, you found a group willing to stay underground with you – or created them out of cardboard, who’s to say – and stuck out life away from the Sun. What happens when you come back?

Truth is, it might be harder than you think. “It’s always the same when you have to face a new situation,” Clot tells IFLScience. “It’s always quicker to adapt to a new situation than to re-adapt when you go back.”

As with moving in, some of those after-effects will be physical. A month after Flamini emerged from her cave life, her eyes had still not returned to normal: her peripheral vision was gone, and she needed sunglasses to protect her pupils from the bright light of the outside. She also walked with a stoop: “The way you walk, the way you move – that changes” in the cave, Clot explains, “because you have to adapt to a new way of doing things.”

But perhaps more challenging will be the psychological effects. More than her posture and vitals, it was Flamini’s erratic behavior that was worrying her team: “What’s happened since she left the cave shows all the signs of post-traumatic stress syndrome,” María Dolores Roldán-Tapia, a neuropsychologist at the University of Almería who examined Flamini after her re-entrance into the world, told the New Yorker. 

“Her survival in the cave was traumatic, even if she entered it of her own free will,” Roldán-Tapia explained. “There’s a lot of data that makes me think what she experienced there was basically negative.”

It is, perhaps, yet more evidence for Clot’s team-effort approach. Asked whether he and his team would go back, he answered immediately: “Yeah, of course. We will – we’re going back in the cave in June.”