underground tunnel complexes found on the moon

from the register: Space boffins believe they may be on the verge of discovering a vast, hidden network of tunnels beneath the surface of the Moon. The tunnels aren't thought to be the work of long-extinct (or perhaps still surviving) intelligent moon aliens, perhaps driven to adopt a subterranean existence by a notional disappearing atmosphere aeons ago. Nor is any involvement by Tibetans, Elvis or others seeking a secluded dwelling far from prying human eyes suspected. Instead, top moonologists think that they may be on the track of "lava tubes" not dissimilar to those of Earth. A lava tube, unsurprisingly, is a tube in solid surrounding rock along which at some point liquid lava has flowed (or is still flowing, in some Earthly cases). When the lava cools and solidifies, there is often an open space left above its surface, forming a tunnel.