But now, after spending nearly three decades on the margins of the space industry, Stone is closer than he’s ever been to proving that caves are the best earthly training ground for exploring space. Backed by a $5-million grant from NASA, he is developing a robot called DepthX that may turn out to be the most advanced autonomous underwater vehicle (AUV) ever. Like its inventor, DepthX is a caver, capable of navigating constrained, obstacle-filled environments. Its theoretical mission, though, is bold even by Stone’s standards: a hunt for extraterrestrial life on the Jovian moon of Europa.