Quicksand

(Daniel Bonn is a physics professor and quicksand expert. He is now based at Ecole Normale Superieure in Paris.)

“One of the things you hear about quicksand is that you shouldn’t move, and that’s true because this liquefies the material and you sink away in it. It’s also true that once you’re in, it’s almost impossible to get out. Grains of clay and sand form a thick layer around your feet. The force you would need to pull somebody out from this at a reasonable speed would actually pull them in too.

So what you have to do is wiggle your leg. Getting water into the mixture allows you to dilute and expand the sand grains and separate them from each other.

Wiggling lets water trickle between your leg and the hole that your leg created. If you do that slowly and for long enough, you will get out.