There you are walking in the desert and before you know it you're sinking into the sand and there's not a thing you can do about it. Quicksand – a favourite motif of legends, novels and movies. But quicksand is no myth. What you have to look out for are the areas of tideland where the sand is so full of water that it's almost liquid. There's no danger of being fully submerged, but you might get stuck – not a good idea when the tide comes in.

So does that mean that desert quicksand is just an invention? Possibly not. Blow enough air into grains of sand in a laboratory and there's your quicksand. The sand is so loose it can swallow a metal ball. So maybe Hollywood has it right. Dry quicksand really does exist.