A multi-frame panoramic view of the inside of a lava tube at Craters of the Moon National Monument in Idaho, USA. Lava tubes are formed when the surface of a lava flow cools and insulates the molten lava beneath; if the flow tube structure empties, a cave-like void remains. Later collapses have formed the two apertures seen in this image, which shows part of a 250m-long feature known as Indian Tunnel. Lava tubes provide good examples of lunar structures which might be used for future Moon bases. The Craters of the Moon geological feature was formed by lava from a series of deep fissures, known collectively as the Great Rift, which crosses the Snake River plain and dates from about 15, 000 years to 2000 years ago. Some of the Apollo astronauts learnt geological skills here.