Wander way out into the middle of Death Valley and you may come across some rocks. Not just any rocks (though there are certainly plenty of them around). Rocks that walk.

Sure, most rocks move when you kick them. But these rocks, found in the middle of an enormous dried-up clay lake bed called Racetrack Playa, seem to move all by themselves, leaving tell-tale trails behind them in the dust. They don't just follow straight paths either. Some zig. Some zag. Some practically do U-turns.

Why? Well that's the million dollar question. No-one, it seems, has seen them move (they move so slowly and so infrequently that watching them would be more frustrating than watching your toenails grow).

Nor is there any evidence that foul play is afoot. The most popular theory is that, after heavy rainfall, the ground gets seriously slippery, and gusts of wind push the stones (and we're not just taking pebbles here) slowly across the ground, some as far as 1,500ft. The prevailing wind direction (southwest to northeast) gives weight to this theory. Others believe the movement is caused when the lake bed is covered by a thin layer of ice, and the stones take off like rocks in a game of curling.