Space is an unforgiving environment. A single mistake can endanger a mission or the lives of an entire crew.

Mindful of these risks, the European Space Agency (ESA) has created a program that challenges astronauts to work together in an unexplored realm that can be as perilous as space: a network of caves half a mile underground in Sardinia, Italy.

An international team of six astronauts from the United States, China, Japan, Spain, and Russia began their descent in early July. Their six-day mission is to establish a base camp while conducting scientific experiments.

“Astronauts adopt the ‘buddy system,’ and both astronaut trainers and instructors repeat the same mantras of ‘slow is fast,’ ‘check your gear, and then trust it,’