Picture this. All over a flat, dried-up lake in famously desolate Death Valley are hundreds of huge rocks, spread randomly. Behind each of them lies an impossibly smooth trail, as if the rocks have been pushed along the ground. But they’ve done that without any animals or humans touching them – and, to the naked eye at least, they don’t appear to be moving at all.
The rocks are called ‘sailing stones’, and they’re best found on Racetrack Playa, a dry lake in California. So, what actually causes them to drift across the valley?
For decades people blamed high winds, magnetic fields and even aliens for the rocks’ tracks. Even geologists were completely mystified, only able to make wild guesses about how the hundreds of stones – some weighing as much as 320kg – moved. But by the mid-2010s, scientists reckoned they’d figured it out, once and for all.
Essentially, the rocks require a very, very rare combination of conditions to move. First, there has to be enough rainfall for the plain to fill with a few inches of water. It then has to get cold enough overnight to form floating ice, which lifts the rocks slightly. The next day has to be particularly sunny, thereby melting the ice in huge sheets. The wind then helps the melting ice slide the heavy rocks along the plain.
When all the ice is gone, voilà! You’re left with the rocks having both moved and left an almost artificially smooth trail behind them. It is technically possible for the human eye to spot the rocks moving during these conditions, but it’s actually pretty difficult. After all, when a landscape is as barren as Death Valley, and there are no reference points for lots of rocks all moving at the same pace, how do you tell? If you want to read up on a recent study about the phenomenon, click here.
In any case, they make for a fascinating visit – if you’re up for the trip, that is. Racetrack Playa is pretty remote. It’s about three and a half hours away from the nearest proper settlement, Furnace Creek, and the journey is only possible by 4x4.
But most important is that if you do make the trip, be a considerate tourist. Recent years have seen the playa and its sailing stones damaged and vandalized. Walking or driving on the wet plain can impede the rocks’ movement, and inscribing your initials on (or even stealing) the rocks is, obviously, a dumbass move. Some people, eh?
While you’re here, did you know that this fire that has been raging in the Asian desert for 50 years?
And have you heard the one about the Dutch town with precisely zero roads?