Perched on a narrow passage, ESA astronaut Marco Sieber stops for a moment to admire the heart of the cave he is set to explore.

His journey into the deep is part of ESA’s CAVES training that prepares astronauts for the challenges of spaceflight both as individuals and as a team. Just like in space, the cave imposes isolation, confinement and limited supplies.

For four days, an international crew of astronauts descended underground to live and work together, cut off from the outside world. Marco shared the expedition with NASA’s Jasmin “Jaws” Moghbeli, Makoto Suwa from Japan’s space agency JAXA and Mohammad Al Mulla from MBRSC, the Emirati space agency.

Marco’s role for the first two days and nights was to scout the 3.5 km long cave in the Italian Apennines, staying focused for hours on end under real exploration conditions. He was the first one into the unknown. The use of artificial light in constant darkness alters the perception of time and of colour.

Beyond his skills as a medical doctor, his fellow explorers describe him as adventurous and humble, always curious to learn new things.

Together with Jasmin, he was in charge of mapping the cave, choosing the best path and reporting their progress to the ground. All roles were swapped halfway through the expedition.

The team of ‘cavenauts’ navigated sloping and uneven terrain, keeping situational awareness at all times. They overcame vertical drops of 20 metres using newly acquired rope climbing skills and organised the campsite that became their home.

Over four dark days, they discovered a new world and new things about themselves and each other.

“I feel privileged to see nature’s beauty in a new and unexpected way, but also to share these unforgettable moments with my crewmates. CAVES has been a great learning experience in team dynamics,” says Marco.

Throughout the expedition, the crew monitored the cave's changing environment and conducted science experiments, including microbial sampling and tracking radon and carbon dioxide levels.

“Like in space exploration, we are reminded that we might reach places where no one else has been before. We have the duty to preserve it and avoid polluting it,” says Marco.

Listen to Marco on our astro chat podcast, learn how the crew prepared for the cave on ESA’s blog and see them in action on our CAVES Flickr gallery.