Stick in the mud

Here's a 2006 photo of a group of kids "playing" in the quicksand that surrounds the Mont Saint-Michel in Normandy. The adults are there, just outside the photo. The web sites I consulted on the topic (actu.fr and wikihow.com) say that it's nearly impossible to sink all the way into quicksand due to the body's natural buoyancy and the fact that most quicksand is not very deep. The quicksand is actually a pocket of water below the sand's surface. When you step on it, you press out the water as your feet start to sink in. As you'd expect, once your feet go under, your weight continues to push water out of the sand surrounding them, creating a suction force that prevents you from wriggling out. The more you fight it, the farther in you go, to a point. Gentle movements that allow water to re-penetrate the sand will work to free you. One leg at a time.

It's easier to free yourself if you're barefooted.