The standard Hollywood “quick sand” is plain water with a layer of vermiculite floating on top. It crops up so often in movies and it’s always one of the hallmarks of a truly bad film – what director has to reach for such a hackneyed plot device, anyway?
However, quick sand is real – it’s just fairly rare and occurs in specific conditions. This is a really amazing video of some tourists (with a competent guide) exploring quick sand near Mont Saint Michel in Normandy. There’s a lot of tidal action there, and that means sand gets saturated and mixed with water as the tide is going out, which gives a brief window in which the sand gets … weird: [The action starts around 0:48]
Isn’t that nuts? Sand is supposed to be solid.