This article originally appeared on VICE Italy.

Could heading deep into the bowels of the earth help train us for life on other planets? This is the idea behind the CAVES (Cooperative Adventure for Valuing and Exercising human behaviour and performance Skills surely the furthest reach an acronym has made in decades) programme, led by the European Space Agency (ESA). The project sends astronauts to live in a cave for six days, with the task of exploring, mapping and conducting scientific experiments and coming back in one piece.

Its whats known as an analogue mission: one carried out on earth to simulate space travel. Immersed in an entirely alien environment, astronauts improve their communication and problem-solving skills and test the tools and technologies they will use in future missions. The long-term goal of CAVES is to lay the foundations for future exploration of the Moon, and, going a bit further, Mars. Caves welcome and protect us: the thinking goes that they will be our first home when we arrive on other planets.

The astronauts climb the walls of the Divaka Jama caves. Image: ESA by A Romeo

The sixth CAVES simulation was completed in September, and took place for the first time in Slovenias Divaka Jama cave, just a few kilometres from the Italian border and 250 metres underground at its deepest point. Six astronauts from five different space agencies lived for six days and six nights in total darkness, at six degrees Celsius and 100 percent humidity.

I had this idea for preparing astronauts to become efficient and reliable members of long-duration flights and explorations,'' says ESA astronaut trainer Loredana Bessone, the brains behind CAVES. I wanted something that allowed me to replicate that particular condition of stress.

The astronauts setting up base camp. Image: ESA by A Romeo

Calling it a simulation is a bit of an understatement. The cave is real and the risks are real, says Bessone. Astronauts have to learn how to cope with the fear.

The cavenauts werent entirely alone in their mission. They were supported albeit from a distance by a group of real speleologists, people who study caves. The logistics of the whole operation were managed by the start up Miles Beyond, which specialises in providing support in extreme environments. Outside the cave we had a team of 25 people ready to intervene, says Tullio Bernabei, a speleology professor and member of Miles Beyond.

The astronauts of the sixth CAVES mission exploring the Divaka Jama caves in Slovenia. Image: ESA by A Romeo

On the 25th of September, the six cavenauts emerged, looking a bit rough from their time underground. The next day, with the sun shining in Divaka Jama, most of them wore sunglasses to protect themselves from the intense light, and hide the signs of a week of extreme training. I was one of the people gathered there to hear what it was like.

Astronaut Joshua Kutryk (middle) using sampling tools before the mission. Image: ESA by A Romeo

VICE: Had you been down into a cave before? Joshua: Kutryk: No, this was my first time. The environment is very dangerous and for many of us it was something completely new. It was great training and it really is as challenging as they describe it.

How did you find the environment? Was it really so alienating? A cave is a great place to experience that prolonged sense of isolation. Even reaching the starting point of the mission, where we set up base camp, was really difficult because we had to go down tens and tens of metres. It took a lot of rope, time and work even just to get started. Its during these simulations that we understand how even the smallest mistake can have terrible effects.

Did you have to protect yourselves from bats?[Laughs] No! Theres no bats that deep down. But there are forms of microscopic life that are definitely fascinating.

Astronaut Alexander Gerst from the ESA during the mission. Image: ESA by A Romeo

VICE: Youre laying the foundations for future extraterrestrial settlements. The future seems both very far and very near. Mars and the Moon have many caves. They are much wider than the ones on Earth up to a kilometre wide and hundreds deep. Imagine what that means you could build a city for hundreds of thousands of inhabitants. It sounds crazy, but its true.

On earth we think of caves as a hostile environment, only because we have the luxury of having an atmosphere that suits us. On other planets, however, caves will be the best places to live. We will have to explore them and to do so we must prepare now.

NASA astronaut Joe Acaba (right) during the mission in Slovenia. Image: ESA by A Romeo

VICE: You look really tired. How did it go? Joe Acaba: Great. CAVES is by far one of the best analogue missions out there to prepare for space travel. You learn to manage your equipment and you understand to what extent your life and that of your team depends on the support of everybody involved.

Did you get any sleep? Definitely. The days were long, the cave was freezing and once I got into my sleeping bag, I fell asleep in a matter of seconds. But we were in a cave, so if someone started to snore, the echo was really loud!

Will you go to the lunar caves? I dont know when I will go back into space. Ive been three times, the last in 2018. But I get really excited thinking about the future, when humanity will go back to the Moon.

NASA astronaut Jeanette Epps during the mission. Image: ESA by V Crobu

VICE: You took part in NEEMO [the NASA analogue mission set in a submarine station] and became a waternaut, and now youre a cavenaut. When will you become an astronaut? Jeanette Epps: [Laughs] I hope very soon! Im not sure when it will be, exactly, but I know that analogue missions like CAVES are helping me get ready. The cave is an extreme environment and it was really alienating to be so deep underground for six days.

How did it feel exploring a cave? You feel the stress. Our main daily goal was to protect our own and the others safety. The exploration was really difficult. It was dark, slippery, hostile down there. When it started raining, things got a lot worse. It was hard but it was a wonderful experience. I learnt to know myself better and to understand certain aspects of my body in certain situations. What I learnt in these six days will be fundamental once I go up there, into space.

Roscosmos cosmonaut Nikolai Chub during the caves mission. Image: ESA by V Crobu

VICE: Had you already taken part in other analogue missions? Nicolai Chub: Yes, I took part in NASAs NEEMO, but it doesnt compare to CAVES. The daily tasks on this cave mission were really demanding and the risks were constant and variable.

What pushes a cosmonaut to go down into a cave? We have to be ready for anything, for any possible event. Even for an emergency landing anywhere on earth. Thats why we train to survive in the desert, in the forest, underwater, at high altitude and even in a cave. It was really a unique mission. Life in there is not normal.

VICE: Had you prepared for the experience? Takuya Onishi: Id received no specific training but when we arrived here in Slovenia we did a ten-day course on the basics of speleology. Before that, nothing. It was a real challenge for me.

How will you use the experience you developed in this mission? You know, one of the biggest problems well have once we get to the Moon or Mars will be radiation. Well have to protect ourselves somehow. Were already thinking about building the first human outposts in caves. This CAVES programme is one of the first bricks of knowledge that will allow humanity to push itself further and further. Always further, to explore the unknown. The caves gave us a home and sheltered us during explorations in the past. They will do so in the future. They will be our first home.

To see more images from the latest CAVES mission, head here.

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The Best Place to Train Astronauts Exists on Earth, But Not Where You'd Think - VICE UK