Lifeboat Foundation

Slight variations in the moon’s gravitational tug have hinted that kilometers-wide caverns lurk beneath the lunar surface. Like the lava tubes of Hawaii and Iceland, these structures probably formed when underground rivers of molten rock ran dry, leaving behind a cylindrical channel. On Earth, such structures max out at around 30 meters across, but the gravitational data suggest that the moon’s tubes are vastly wider.

Other satellites had found the openings of large lunar lava tubes and caves.

• The Lunar Reconnaissance Orbiter has now imaged over 200 pits that show the signature of being skylights into subsurface voids or caverns, ranging in diameter from about 16 feet (5 meters) to more than 2,950 feet (900 m), although some of these are likely to be post-flow features rather than volcanic skylights.