What is quicksand?

How to get out of quicksand : Slowly float out. The best way to do this is to try to increase your surface area by leaning back into the quicksand and slowly moving your legs to liquefy the water around them. Don't kick wildly. If you're very close to solid land, sit down on it and slowly work your feet or lower legs free. The way to do it is to wriggle your legs around. This creates a space between the legs and the quicksand through which water can flow down to dilate [loosen] the sand,"

scary to fall into, and knackering to get out of!