NASA/GSFC/Arizona State University Beneath its pock-marked, craggy exterior, the Moon is thought to harbor a network of caves and tunnels from its volcanic past. Now, NASA has selected a mission to go investigate a promising window into our space rock’s subterranean secrets. Gaping up at outer space, as if mugging for the orbital satellite cameras, is the Marius Hill Pit, a hole on the lunar surface that’s thought to be up to 150 feet deep and up to 300 feet across. Scientists believe it’s an example of a skylight — the collapsed roof of an underground lava tube, which forms...