The European Space Agency (ESA) says it wants humans to one day take long-term shelter in underground caves on the Moon.

While the surface of the Moon has been explored, walked, and even golfed upon, relatively little is known about what lurks below the surface. ESA wants to explore beneath this surface, particularly pits that planetary geologists have suggested could be caused by the collapse of lava tubes when lava flowed under the surface over a billion years ago.

The lunar maria (large, dark plains) were caused by huge flows of basaltic lava, almost exactly the same type you would see in Hawaii, that flooded out after impacts from various types of space rock.

“Exploring and mapping these tubes could provide new information about the Moon’s geology, but they could also be an interesting option as long-term shelter for future human visitors to the Moon,” Francesco Sauro, director of ESA’s PANGAEA planetary geology astronaut training, said in a statement.

“They would shield astronauts from cosmic radiation and micrometeorites and possibly provide access to icy water and other resources trapped underground.”

_{NASA is exploring the idea, too. "This week our planetary scientists are in Hawaii, exploring lava tubes to better understand similar structures on the Moon and Mars," they wrote on Twitter. "These caves might contain evidence of ancient life on Mars or serve as shelters on the Moon."}

The agency has put out a call for ideas on how to explore areas under the lunar maria, and what else such missions could investigate. ESA say they're looking for ideas for missions beyond just how to access and navigate the caves, e.g. ways that a communication system between underground caves and the outside world could be established.

“Mission concepts may be based on a single rover or a distributed system of satellite, robotic or rover systems that operate together,” Loredana Bessone, who is leading the hunt for ideas as head of analogue field testing and exploration training at ESA, said in a statement.

“Either way, we are looking for systems that would land on the lunar surface, identify and access a cave and contribute to the scientific exploration of the Moon.”