Quicksand used to be a common plot device in adventure films, with people inadvertently stepping into it and being slowly sucked under while others were helpless to extricate them. But it turns out that what is more likely to happen is that as long as you don’t thrash around, you will sink about halfway and stop. But getting yourself out is not easy and requires considerable force. This is what a paper in the 29 September 2005 issue of Nature (vol. 437, p. 635) says.

The natural quicksand that we study here consists of fine sand, clay and salt water. Rheometrical tests (Fig. 1a, b) reveal its extreme sensitivity to very small variations in stress. At rest, its viscosity slowly increases with time— a behaviour characteristic of clays2,3. This reflects the formation of a fragile colloidal gel that has a random, delicately balanced structure. At higher stress, a spectacular liquefaction of the material takes place: the steady state viscosity changes by several orders of magnitude for a variation in stress of less than 1%. The higher the stress, the more liquid the quicksand becomes, so movement by a trapped body causes it to sink in deeply.

Why is it that, once sunk in quicksand, it is so difficult to escape? Because the apparent viscosity of quicksand increases after the initial stress-induced liquefaction, unlike that of clay or sand alone. After liquefaction, the quicksand is seen to segregate into a water-rich phase and a sand-rich one. The apparent viscosity increase is therefore due to the formation of sand sediment, which has a very high volume fraction (φ ≃ 0.8) and viscosity. It is the difficulty of moving this densely packed, wet sand that leads to trapping. Water must be introduced into the compacted sand to liquefy it, which requires huge forces: to introduce water at a speed of 1 cm s^-1, say, a pressure of 10⁶ pascals (Pa) is needed, assuming a typical sand-pore size of 10 microm. To pull out a foot at this speed, a force of some 10⁴ newtons is required about that needed to lift a medium-sized car.
…

The most important conclusion from the sinking experiment is that it is impossible to sink beads with a density of 1 g ml^-1: they continue to ‘float’. As this is typically the average density of humans and animals, any unfortunate victim should sink halfway into the quicksand, but could then take solace from the knowledge that there would be no risk of being sucked beneath the surface.

So how do you get out if you happen to get trapped in quicksand? The main thing is to keep calm and not thrash around because that can actually make you sink, at least temporarily, as the researchers found using aluminum beads placed in a container that had a quicksand-like mixture.

If the whole system is mechanically shaken to mimic movement in the quicksand, the results agree with the rheological findings. At small amplitudes (acceleration a<3.16 m s^-2), the bead stays afloat; however, liquefaction occurs at larger amplitudes and the resulting low viscosity causes the bead to fall to the bottom of the container. Liquefaction is so rapid in this case that sedimentation does not have time to occur.

But even after you sink, if you stop thrashing around, the quicksand can kind of vomit you out, since your density is less than that of the quicksand. This video tells you what you should do

But the best thing to do is to avoid quicksand altogether.