oh yeah I know all about it. Technically, quicksand is sand suspended in water. It has non-Newtonian properties, sand particles are able to slowly move around like a fluid, but if they move too fast they lock up. So you can sink into it slowly, if you struggle too fast it locks around you. But more commonly the term is broadened to include deep mud pits, usually clay silt, and peat bogs. All of them are denser than water, so there’s no real danger of sinking under, you’ll float around chest deep. Though you can push yourself under.