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.