The vast field of Salisbury Plain in Wiltshire, southern England, is a stretch of verdant land steeped in history. It's here where the ancient neolithic site of Stonehenge sits, an inscrutable pile of rocks wrapped in mystery. And it's here, or at least in the town of Warminster, where some of the U.K.'s most impassioned UFO watchers gather atop nearby Cradle Hill in the belief that they can signal extraterrestrial spaceships.