From an estate agent’s perspective, the lunar surface has little to recommend it. Its none-too-metaphorical lack of atmosphere means it is bombarded by meteorites, cosmic radiation is unrelentingly harsh and temperatures range from lows of -246°C to highs of 121°C. Subsurface lunar caves, on the other hand, with stable temperatures and cover from projectiles, would be much more enticing habitats. The only question is, do any exist?

In a paper published in Nature Astronomy, Lorenzo Bruzzone and Leonardo Carrer, at the University of Trento, and their colleagues, offer a definitively affirmative answer. They provide evidence of a cave about 80 metres long and 45 metres wide at the bottom of a 135-metre-deep pit in the Sea of Tranquility. The neighbourhood is a historic one: plains which surround the pit are the site of humanity’s first lunar landing. Though the site’s selenological (the lunar equivalent of geological) origins are unclear, it may be the remains of a subsurface tube forged by a river of lava. When the tube’s ceiling caved in—perhaps after a meteor impact—a giant pit (with its associated caves) was all that survived.

The researchers discovered the annexe by analysing data from the Lunar Reconnaissance Orbiter (LRO), which has been circling the Moon since 2009. Aboard the LRO is a radar instrument primarily designed to hunt for water ice under the Moon’s surface. The instrument also collected data about the shapes of pits such as the one in the Sea of Tranquility. Although the images the radar instrument created have been available since it broke down in 2011, says Dr Bruzzone, nobody had previously looked at them in the right way to be able to spot a cave.

That changed when the team compared the pictures with satellite images of Earth created using radar technology similar to the LRO’s. That helped them spot the telltale signature of a cave, as well as to work out the different radar signals given by different subsurface geometries. The team then ran simulations to compare the data from LRO with those expected from pits of varying shapes and sizes. The best match for the Sea of Tranquility data, it turned out, was a pit with a spacious hidden cave. Importantly, its entrance is accessible. “We found a front door,” says Dr Carrer. This means that it could be explored in future lunar missions.

Such exploration could be useful for more than house-hunting. Subsurface lunar caves are thought to contain valuable resources that future Moon-dwellers might need to set up permanent bases. Water, for example, could be drunk as well as used as a potential source of fuel and oxygen. It has already been found in both solid and liquid form on the lunar surface, but more may be hidden beneath.

The caves would also possibly shed light on how the Moon came to be. Just as studying Earth’s inner layers reveals clues about conditions on the planet when each layer was formed, studying the walls of a pristine lunar cave could paint a fuller picture of the Moon’s formation. Scientists could use them to map the extent of volcanic activity on the Moon, perhaps, or determine the composition of the lunar core. All that’s needed is a tenant with vision.

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