Let me begin by saying that hopefully the CGS' esteemed leader will be able to clear a spot from his busy schedule to furnish us with an accurate answer. There is certainly some evidence to suggest there might be caves, or caverns on the moon: http://science.nasa.gov/science-news/science-at-nasa/2010/12jul_rabbithole/ http://www.nasa.gov/content/goddard/lunar-pits-could-shelter-astronauts-reveal-details-of-how-man-in-the-moon-formed/#.VEgyI_mktGY The mechanism for the formation of these features is not well understood. Erosion or weathering by liquid (water) seem unlikely. It seems the primary conjecture is impacts opening up ancient lava tubes, and exposing the voids therein. Which seems plausible, at least to a layman, such as myself. Strange things and formations do appear in space though, you need only look at the comet which the Rosetta mission is currently orbiting, and soon to send a lander to, 67P Churyumov-Gerasimenko (known as 67P, or 'Churi Guri', for obvious reasons) https://www.flickr.com/photos/europeanspaceagency/sets/72157638315605535/ Or Saturn's moon, Hyperion: http://solarsystem.nasa.gov/multimedia/display.cfm?Category=Planets&IM_ID=4143 Both have formations that seem hard to explain as a consequence of impacts alone. Even without the magic of The Traveller, Bungie have pretty much free licence it seems. The truth of our solar system is always stranger than fiction. This is why I can't wait for the upcoming missions to Ceres & Pluto amongst others. Who know what amazing stories those rocks will have to tell!