Cave is totally dark - except 'own' lights; photo from third exploration day.

ESA’s cave training prepares astronauts to work as an international team in
real exploration conditions. Sending astronauts underground to survive and
explore Sardinian caves in Italy for almost a week is just one element of
their long training.

The Sardinian caves are isolated from the outside world. The astronauts need
to get used to confined spaces, minimal privacy, technical challenges and
limited equipment and supplies for hygiene and comfort - just like in space.