Unforgiving sun, punishingly hot days, freezing nights, no water, and a name like Death Valley keep people away from this ultra-dry section of California. Those who do venture there notice that sometimes, in some places, rocks just move. They don't do it a lot, and they never do it when anyone is watching. Make no mistake, though, they do it. They leave behind long, even grooves in the packed dust to prove it.
When walking through Racetrack Playa, a dried up lake bed in Death Valley, it's not unusual to see these moving rocks, with their long tracks stretched out behind them. They're never moved into any kind of structure, so it's unlikely that humans are doing - although scientists have not ruled out some kind of prank. The occasional rains in southeastern California can flood the lake beds. Rains can create small rivers that go for miles, so parts of the desert can flood even when there isn't a cloud in the sky. But rocks don't float, so how do they manage to move themselves?
Some scientists believe that, although the rock isn't picked up by the water, it is driven by the wind. Winds across the desert can be intense, and if the lake bed floods and the packed dust becomes slippery mud, the rock might be blown or washed one direction, leaving a long groove behind it. The wind might also cause the water to chill to freezing. Sheets of ice would corral the wind even more effectively, making it stronger and allowing it to push rocks across the desert. The newest suggestions combines both ideas. Rainwater would cause the playa to flood, and ice would be formed by the wind and dropping temperatures. That ice would freeze around the stone. As water freezes, it becomes less dense, and floats. Since it freezes around the stone, a small ice berg would lift the stone up slightly. Then the wind would push the ice raft along, letting the stone scrape along the mud.
Lab tests support this theory. Scientists, including Ralph Lorenz of Johns Hopkins University who came up with the ice raft theory, would like to install time-lapse cameras on the Racetrack Playa, to try to capture the movement of the rocks. No word yet on what the scientists would do if it turned out to be poltergeists.
Image: Nasa/GSFC/Cynthia Cheung