Darkness as a training ground
At first glance, a network of underground passages seems to have little in common with an orbital station or a lunar habitat. But the European Space Agency has turned cave environments into a core training asset through its CAVES programme — short for Cooperative Adventure for Valuing and Exercising human behaviour and performance Skills. The concept is straightforward: place a crew of astronauts in a natural cave system for several days, strip away daylight, familiar technology and easy communication, and observe how they perform.
The conditions that make caves challenging — limited resources, spatial disorientation, physical fatigue and enforced reliance on teammates — mirror in meaningful ways the psychological and operational pressures of long-duration spaceflight. This parallel is not accidental; it was the founding rationale behind the programme's design, developed in close partnership with expert cavers and behavioural scientists.
A living laboratory for crew dynamics
Each CAVES session brings together an international crew drawn from ESA member states alongside astronauts from partner agencies including NASA, JAXA, Roscosmos and ISRO. Their tasks underground are deliberately varied: mapping unexplored tunnels, collecting geological samples, managing simulated emergencies and maintaining structured communication with a surface support team acting as a stand-in mission control.
Researchers embedded in the exercise focus less on raw physical performance and more on the subtle mechanics of group behaviour. How are decisions made under time pressure? Who steps into a leadership role naturally, and when does that shift? How does a team manage conflict, or the slow erosion of concentration that comes with cumulative fatigue? The answers feed directly into crew preparation protocols for the International Space Station and, further ahead, for the lunar surface operations envisioned under the Artemis programme.
The caves also carry independent scientific value. Some formations host extremophile microorganisms or unusual mineral deposits whose study informs ongoing research into the potential habitability of environments like the Martian subsurface or the ice-covered oceans of Jupiter's moon Europa.
Rethinking what it takes to fly in space
CAVES reflects a broader shift in how ESA approaches astronaut selection and preparation. The agency now places explicit emphasis on what it calls non-technical skills — communication across cultural differences, stress regulation, situational awareness and adaptability — treating them as equally essential to mission success as any procedural competency.
Astronauts who have completed the programme consistently describe it as among the most demanding and formative experiences of their training pipeline, in large part because it removes them from the reassuring framework of simulators and technical checklists they usually rely on.
As Europe works to secure a stronger role in international human exploration decisions — from ISS operations to future Gateway lunar orbit missions — the quality of its astronaut preparation remains one of its most tangible assets. Investing in the human element, it turns out, may matter just as much as investing in the hardware. Sometimes, the road to the stars runs underground.
