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Quicksand forms in saturated loose sand when the sand is suddenly agitated. When water in the sand cannot escape, it creates a liquefied soil that loses strength and cannot support weight. Quicksand can form 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 in a 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."
Quicksand itself is harmless; a human or animal is unlikely to sink entirely into quicksand due to the higher density of the fluid. Continued or panicked movement may cause the victim to sink deeper, leading to belief that quicksand is dangerous. Because it increasingly impairs human locomotion, it allows harsher elements such as sunlight, dehydration, carnivores, omnivores, hypothermia or tides to harm a trapped person. Quicksand may be escaped by slow movement of the legs in order to reduce viscosity of the fluid, and rotation of the body so as to float in the supine position.
People falling into (and, unrealistically, being submerged in) quicksand or a similar substance is a trope of adventure fiction, notably in 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.
Media related to Quicksand at Wikimedia Commons