Come see the Mysterious Sliding Rocks of Death Valley.

Scattered across the flat surface of Racetrack Playa, far from the edges of the surrounding mountains, are lonely boulders and smaller pieces of rock, known as the mysterious sliding rocks of Death Valley. Stretching behind many of these famous rocks are trails - some short, some long, some straight, some curvy. These rocks gouge furrows into the desert floor as they slide across the playa surface, yet no living person has ever witnessed them move!

What makes these rocks, some as large as 705 lb (320 kg), skid as much as 2890 ft (880 meters) across the playa surface?

Racetrack Playa is an almost perfectly flat dry lake bed between the Last Chance Range to the west and the Cottonwood Mountains to the east. During heavy rains, water washes down from nearby mountain slopes onto the playa, forming a shallow, short-lived lake. Under the hot Death Valley sun, the water quickly evaporates, leaving behind a layer of soft mud. As the mud dries, it shrinks and cracks into a mosaic of interlocking polygons.

The shallow furrows that form the "trails" of the sliding rocks are clues that suggest the stones move only when the playa surface is soft and wet. Once an object is put into motion on a very low-friction surface, like mud, it may move pretty far before it stops.

Some thought that gravity was the cause and that the rocks might be sliding downhill on a very shallow slope; however, this hypothesis was abandoned when it was shown that the northern end of the playa is several centimeters higher than the southern end, so many rocks were actually moving uphill!

Without witnesses to the sliding rock phenomenon, it's been difficult to prove exactly what makes the Racetrack rocks move. Research has also been hampered because traces left behind by sliding rocks are fleeting. Small rock trails can be washed away by a single rain storm. Even trails gouged into the playa by the largest boulders last no more than seven years.

Researchers noticed that although some trails change direction, most generally headed from southwest to northeast. This is consistent with the direction of the prevailing winds. One recent study used a high-tech approach in an attempt to solve the mystery of the sliding rocks. Detailed measurements using GPS were made of over 160 sliding rocks and their trails.

They found that the longest, straightest trails were heaviest in the southeastern part of Racetrack Playa, where the wind was being channeled through a low point in the mountains, forming a wind tunnel. In the central part, two natural wind tunnels focused their energy from different directions. It's here that rock trails were the most elaborate. So, they surmised that strong gusts of wind and swirling dust devils, in combination with a slick playa surface, may set even the heaviest the rocks in motion.

The Racetrack is accessible by a 28-mile dirt road that begins near Ubehebe Crater. This road is often very rough, and a high clearance vehicle is recommended. The surface of the playa is very fragile, and off-road driving is strictly prohibited. Avoid walking on the playa when the surface is wet. To see the moving rocks, drive to a parking area at the southern end of the Racetrack, and walk at least a half-mile toward the southeast corner of the playa.