We’re celebrating 20 years of Earth Science Picture of the Day during the month of September! Today’s photo features a popular EPOD from the past. Thanks to all of our followers (on the blog, Facebook, Instagram and Twitter) for supporting us. Thanks also to all of you who’ve submitted your photos. We’re most appreciative. This EPOD was originally published March 5, 2002.

Provided by: Joe Orman

Summary author: Joe Orman

In a remote corner of California's Death Valley National Park, on the floor of a dry lakebed called Racetrack Playa, can be seen the geological oddity of the sliding stones. While the force of gravity easily explains how dozens of rocks -- weighing up to 700 pounds -- could have fallen from the adjacent hillsides onto the margins of this long-vanished ancient lake, it cannot explain how the rocks have moved far out onto the perfectly flat surface.

No one has ever seen the rocks actually move, but the evidence is indisputable: distinct tracks in the hard clay surface, some extending for hundreds of feet. Some of the grooves are arrow-straight, while others curve, take sharp turns, or even make complete loops.

For decades scientists have plotted the tracks, analyzed the soil, and charted the weather in an attempt to solve the mystery. All of the theories involve extremely strong winds, but they differ on the exact physical process that allows the rocks to overcome friction. Is the answer mud, water, ice -- or some combination of these? What is clear is that the Racetrack is a unique opportunity for science to explain what at first appearance seems to be completely inexplicable.

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