For decades, researchers have been trying to figure out how hundreds of rockets seem to just sail across the sand in Death Valley. Some of those rocks weigh up to 700 pounds. The area where the sailing stones have been observed is the part of Death Valley known as Racetrack Playa.

Death Valley Sailing Stone

Sailing stone: In the right place at the right time

Researchers set up an experiment in the winter of 2011. After about two years, they went back to Death Valley and found a pond that was several inches deep, reports Daily Digest News. Not long after they observed the pond, the rockets began to move. They thought five to ten years would have passed without anything moving, but researchers said they had some good luck and just happened to be there at the right time to see it firsthand.

They said it takes several things in order for the sailing rocks to move. First, they said the playa fills with water. Then overnight, the pond freezes in thin sheets of ice after the temperature drops. Then on sunny days, the ice begins to melt, breaking up into large, thin pieces that float. Then winds carry them across the playa, and they push the rocks ahead of them.

Slim chances of seeing sailing rocks

Because all of these conditions must be met and result in so many specific circumstances, it is unsurprising how rare it is for rocks to move. In fact, researchers say the last time they believe the sailing rock phenomenon occurred was in 2006. In their estimates, the rocks in Death Valley “move only about one millionth of the time,” according to Johns Hopkins University researcher Ralph Lorenz.

He added that the frequency of rock movement seems to have been declining since the 1970s because it requires very cold temperatures at night. The researcher said that climate change is likely causing this decline.

Researchers published the results of their sailing rocks study in the journal PLOS ONE.