Mystery of Death Valley's moving rocks solved

California - For years scientists have theorised about how large rocks - some weighing hundreds of pounds, zigzag across Racetrack Playa in Death Valley National Park, leaving long trails etched in the earth.

Now two researchers at the Scripps Institution of Oceanography have photographed these "sailing rocks" being blown by light winds across the former lake bed.

Cousins Richard Norris and James Norris say the movement is made possible when ice sheets that form after rare overnight rains melt in the rising sun, making the hard ground muddy and slick.

The phenomenon doesn't happen often because it rarely rains in the notoriously dry desert valley.

Their report says the rocks move very slowly, about 4.6 meters per minute.

The findings were published on Wednesday in the online journal PLOS ONE.