Quicksand forms in saturated loose sand when the sand is suddenly agitated. When water in the sand cannot escape, it creates liquefied soil that loses strength and cannot support weight. Quicksand can be formed in standing water or in upwards flowing water (as from an artesian spring). In the case of upwards flowing water, seepage forces oppose the force of gravity and suspend the soil particles.
The saturated sediment may appear quite solid until a sudden change in pressure or shock initiates liquefaction. This causes the sand to form a suspension and lose strength. The cushioning of water gives quicksand, and other liquefied sediments, a spongy, fluidlike texture. Objects in liquefied sand sink to the level at which the weight of the object is equal to the weight of the displaced soil/water mix and the submerged object floats due to its buoyancy.
Liquefaction is a special case of quicksand. In this case, sudden earthquake forces immediately increases the pore pressure of shallow groundwater. The saturated liquefied soil loses strength, causing buildings or other objects on that surface to sink or fall over.
Quicksand is a non-Newtonian fluid: when undisturbed, it often appears to be solid ("gel" form), but a minor (less than 1%) change in the stress on the quicksand will cause a sudden decrease in its viscosity ("sol" form). After an initial disturbance — such as a person attempting to walk on it — the water and sand in the quicksand separate and dense regions of sand sediment form; it is because of the formation of these high volume fraction regions that the viscosity of the quicksand seems to decrease suddenly. Someone stepping on it will start to sink. To move within the quicksand, a person or object must apply sufficient pressure on the compacted sand to re-introduce enough water to liquefy it. The forces required to do this are quite large: to remove a foot from quicksand at a speed of .01 m/s would require the same amount of force as "that needed to lift a medium-sized car."
Because of the higher density of the quicksand, it would be impossible for a human or animal to completely sink in the quicksand, though natural hazards present around the quicksand would lead people to believe that quicksand is dangerous. In actuality the quicksand is harmless on its own, but because it greatly impedes human locomotion, the quicksand would allow harsher elements like solar radiation, dehydration, carnivores, hypothermia or tides to harm a trapped person.
The way to escape is to wiggle the legs as slowly as possible in order to reduce viscosity, to try spreading the arms and legs far apart and lying supine to increase the body's surface area, which should allow one to float.
In fiction 
People falling into (and, unrealistically, being submerged in) quicksand or a similar substance is a trope of adventure fiction, notably movies. According to Slate, this gimmick had its heyday in the 1960s, when almost 3% of all films showed someone sinking in mud, sand, or clay, but it has since fallen out of use. The proliferation of quicksand scenes in movies has given rise to an internet subculture scene dedicated to the topic.
In music 
Pete Seeger's song "Waist Deep in the Big Muddy" mentions someone drowning after getting stuck in quicksand. There is also a heavy rock band that goes by the name Quicksand (band), which was formed in 1990.
See also 
- Khaldoun, A., E. Eiser, G. H. Wegdam, and Daniel Bonn. 2005. "Rheology: Liquefaction of quicksand under stress." Nature 437 (29 Sept.): 635. doi:10.1038/437635a
- Discovery Channel. MythBusters. Season 2. "Killer Quicksand." October 20, 2004.
- Bakalarfor, Nicholas (September 28, 2005). "Quicksand Science: Why It Traps, How to Escape". National Geographic News. Retrieved October 9, 2011.
- Engber, Daniel (23 August 2010). "Terra Infirma: The rise and fall of quicksand.". Slate. Retrieved 23 August 2010.
Media related to Quicksand at Wikimedia Commons