Did you know that there are lava tubes on the Moon? NASA's Lunar Reconnaissance Orbiter (LRO) beamed back images of caverns hundreds of feet deep in 2010 -- beckoning scientists to follow. "They could be entrances to a geologic wonderland," said Mark Robinson of Arizona State University, principal investigator for the LRO camera. "We believe the giant holes are skylights that formed when the ceilings of underground lava tubes collapsed." Japan's Kaguya spacecraft first photographed the enormous caverns in 2009. Now the powerful Lunar Reconnaissance Orbiter Camera (LROC, the same camera that photographed Apollo landers and astronauts' tracks in the moondust) has given us enticing high-resolution images of the caverns' entrances and their surroundings. Back in the 1960s, before humans set foot on the Moon, researchers proposed the existence of a network of tunnels, relics of molten lava rivers, beneath the lunar surface. They based their theory on early orbital photographs that revealed...