Visitors will notice something incredibly strange at Racetrack Playa, a lake bed located 3,608 feet above sea level. The playa is named after the "racetracks" left by stones that mysteriously skid across its surface. Sliding slowly without any intervention for distances as long as 1,500 feet, the sailing stones have never been filmed in motion, yet they're definitely on the move. The strange phenomenon only happens every few years, and the tracks are left for several years for visitors to ponder. One hypothesis is that winds -- which can howl at up to 90 mph -- push the stones across the slick mud left as the lake bed dries up after a downpour. A second similar hypothesis poses that after a thin sheet of rain falls over the playa and night temperatures drop to below freezing, a layer of ice forms across the surface of the lake bed and the stones slide across the ice with the wind.