Quicksand is basically just ordinary sand that has been so saturated with water that the friction between sand particles is reduced. The resulting sand is a mushy mixture of sand and water that can no longer support any weight. It is rarely deeper than a few feet. If you step into quicksand, it won't suck you down. However, your movements will cause you to dig yourself deeper into it.

Quicksand is not a unique type of soil; it is usually just sand or grainy soil mixed with water. Basically the sand is floating on water. It can occur anywhere under the right conditions. Quicksand is created when water saturates an area of loose sand and the ordinary sand is agitated. When the water trapped in the batch of sand can't escape, it creates liquefied soil that can no longer support weight.

There are two ways in which sand can become agitated enough to create quicksand: Vibration tends to enhance the quickness, so that what is reasonably solid initially may become soft and then quick. The vibration plus the water barrier reduces the friction between the sand particles and causes the sand to behave like a liquid. To understand quicksand, you have to understand the process of liquefaction. When soil liquefies, as with quicksand, it loses strength and behaves like a viscous liquid rather than a solid. Liquefaction can cause buildings to sink significantly during earthquakes.

Contrary to popular belief, quicksand is not found in the desert. Deserts lack the one key ingredient that quicksand must have: water. Places where quicksand is more likely to occur include: riverbanks, beaches, lake shorelines near underground springs and in marshes.

- Answer provided by Ask Dr. Universe