Caves are often measured by the structures they could hypothetically contain. Fantastic Pit in Ellison's Cave, the deepest in the U.S., could engulf the Washington Monument. Plunging 586 feet beneath the surface of Pigeon Mountain in Georgia, the pit is open year-round, but should only be entered by experienced climbers. At least three people have died of hypothermia after being trapped in the pit and another fractured his skull and femur after plummeting 30 feet in 2013.