The Moon’s surface is hosting an increasing number of experiments, robots, and most recently tardigrades. The many underground tunnels that lie beneath the Moon’s surface, however, remain virtually unexplored.

On Wednesday, the European Space Agency (ESA) put out a call for ideas about how to navigate, map, and study these Moon caves in future robotic missions. The campaign is one of many recent projects focused on the exploration of so-called lunar lava tubes, especially their potential as sites for future Moon bases fit for long-term habitation by humans.

The caves could “shield astronauts from cosmic radiation and micrometeorites and possibly provide access to icy water and other resources trapped underground,” said Franceso Sauro, director of ESA’s PANGAEA planetary geology astronaut training, in a statement.

Scientists think lunar lava tubes formed billions of years ago, when molten lava was still erupting on the Moon. Imagery of the lunar surface suggests that these ancient basaltic channels hardened into sublunar caves that may stretch for miles, similar to lava tubes on Earth.

In addition to ESA’s campaign, NASA is developing a “Moon Diver” robot designed to rappel into the deep pits left by ancient lava eruptions. The Search for Extraterrestrial Intelligence Institute has also tested out a drone in Icelandic lava tubes with the aim of eventually flying them in caves on the Moon and Mars.