When a team of scientists found proof of the largest cave on the Moon, their first instinct was to think of it as a shelter for humans.

The deepest known pit on the Moon could open the door to an astronaut base, located roughly 400 kilometres away from the Apollo 11 landing site.

A radar instrument orbiting the Moon showed evidence of a cave with an entrance at least 55 metres wide in the lava fields of Mare Tranquillitatis. The images revealed overhanging walls 75 metres deep and a cave conduit extending over 100 metres below the lunar surface. 

This underground highway could be a lava tube – a tunnel created after the cooling down and hardening of hot streams of lava. 

Spectacular view of the Mare Tranquillitatis pit crater revealing boulders on a smooth floor. Credits: NASA/GSFC/Arizona State University

“There are hundreds of caves on the Moon, but this is the most striking one we have detected to date. We instantly realised it had a remarkable depth, and it looked accessible at the same time,” says Riccardo Pozzobon, co-author of the paper published by Nature Astronomy and instructor in ESA’s PANGAEA geology training for astronauts.

Through the darkness

It was an instrument not designed for the task that provided clues about the cave conduit. A miniature imaging radar onboard NASA’s Lunar Reconnaissance Orbiter called Mini-RF sent the data in 2010. Nobody noticed it before, but later on the team used innovative analysis techniques that revealed new information and provided the first direct proof of the cave.

Images from the Mini-RF instrument onboard the Lunar Reconnaissance Orbiter, including polarimetric decomposition, stereo observations and 3D radar simulations. Credits: L. Carrer et al., from paper “Radar evidence of an accessible cave conduit on the Moon below the Mare Tranquillitatis Pit“.

“We got a lucky pass over Mare Tranquillitatis. The spacecraft was orbiting the Moon exactly over the large pit, giving us details of its interior shape from different angles. We could see through the darkness like never before thanks to the radar images,” explains Riccardo. 

The team performed 3D radar simulations of possible geometries of the pit and the cave conduit, including piles of debris, sharp rocks and fine lunar dust.  

The discovery of the massive interior of this lunar skylight suggests that it could be a promising site for a lunar base. 

Two models representing the best-fitting scenario on the left, together with 3D models of the conduit with a different floor slopes on the right. Credits: L. Carrer et al., from paper “Radar evidence of an accessible cave conduit on the Moon below the Mare Tranquillitatis Pit“.

Protection for a human habitat

Some caves could offer enough space and protection  for a lunar habitat. Like on Earth, temperatures inside lunar caves remain stable with slight variations, and their roofs shield against the harmful cosmic and solar radiation and the impact of small meteorites hitting the surface.

The low gravity on the Moon, about one sixth of Earth’s, allows for larger caves and less collapses. Stable lava tubes could be turned into well-protected habitats.

“Lunar caves, skylights and collapse pits are doorways to the underworld and a great target for human exploration,” says Riccardo.

Cave exploration with ESA astronaut training. Credits: ESA–A. Romeo

“Underground spaces may also host significant water ice deposits and volatiles – for human consumption and for producing hydrogen fuel for the ride back to Earth,” adds the planetary geologist. Volatiles are those delicate chemical components that under certain conditions would just disappear.

From a scientific point of view, a peek into the abyss would be like going back in time. The early chapters of the Moon’s history book could be hidden in ancient rocks below the surface. 

“The deeper you go, the more access we would have to the early stages of the Moon, when it formed around four and a half billion years ago. Lunar caves could also reveal clues about what happened on Earth,” says Riccardo. 

Skylight opening on a huge lava tube in the Marius Hills region on the lunar near side. Credits: NASA/Lunar Reconnaissance Orbiter Camera (LROC)/Science Operations Center (SOC)

Exploration wish list

A complete survey of all known lunar pits would allow planetary geologists to identify the most promising caves for science and human lunar bases, in environments protected from cosmic radiation and with stable temperatures.

“If humankind wants to live and work on the Moon, we need to know it better. We need a fresh perspective to make the difference in future exploration,” says Riccardo. The team behind the paper believes there is a need for a dedicated spacecraft in Moon orbit with a resolution able to map the interior of lunar pits, together with robotic missions on the surface. 

In 2020 ESA ran a competition amongst European companies for ideas to detect, map, and probe caves of the Moon – the SySNova study. The study gave rise to futuristic and bold ideas to descend rovers in Moon skylights. The final step was an early feasibility study of a robotic mission to explore a lunar pit in the Marius Hills. 

In the meantime, ESA keeps training spacefarers to explore caves on Earth. The CAVES course teaches astronauts to work safely and effectively as multicultural and multidisciplinary teams by exploring underground systems in Europe.

The next edition of the course is planned for the fall of 2025. “ESA is developing a novel edition which will increase the crew’s autonomy, as well as their ability to plan safe underground expeditions,” says Loredana Bessone, ESA CAVES course director and project lead. 

“Space exploration will require future crews to become surface explorers again. CAVES is evolving to give them just the right skills,” she concludes. 

Descent into the abyss. Credits: ESA–A. Romeo.