What is quicksand? Quicksand is nothing more than a soupy mixture of sand and water; the term "quick" refers to how easily the sand shifts when in this solid-liquid state. You are likely to find quicksand around riverbanks, lake shorelines, marshes, beaches, near underground springs or any place where an uprising of water over-saturates and agitates the sand.

Sand is actually an interesting medium. Whether it is dry or saturated, it is easy to move around and objects can sink in it. However, when the sand is wet but not saturated, it is stiff and we can drive over it with cars (and tanks) or even better we can build castles with it on the beach. Why is it that the sand behaves so differently with a little bit of water? It has to do with a force imposed by capillaries on the water. The force is best explained by putting a capillary tube (a very, very thin straw) in water; the water begins to rise in the tube. The wider the tube the smaller the rise and the smaller the force. The spaces between the sand grains act similarly to capillaries. When a small amount of water is put in the sand, it will go to the smallest pores and the "capillary" force of water pulls the grains together. When more water is added, the larger pores fill up and the force keeping the grains together becomes smaller. The force disappears when all pores are filled with water allowing the grains to shift when pushed, by stepping on it or by an earthquake. You might remember the San Francisco earthquake a few years back, where the houses that had the greatest damage were build on the saturated sandy deposits.

Now back to the movies. Can a character in a movie really drown in quicksand? The answer is no. The scientific proof is easy and is directly related to the physical law that the upward force on a body immersed in a liquid is equal to the weight of the volume displaced by the body. Dry sand itself weights approximately 1.6 times as much as water. If the pores are filled with water then the sand/water mixture weighs twice as much as water. The character in the movie, just like you and me, weighs approximately the same as water at the same volume. Now if we apply the physics law mentioned above, it is not that difficult to calculate that when half of the character's body is in the water, the upward force is equal to the force imposed by the body weight. Some people (college students of course) have tried to prove this by actually jumping into quicksand (with a rope round their body in case the theory failed). They did not sink any further than their waist into the mud.

This is not to say that quicksand is not dangerous! Mudslides are also triggered by quicksand conditions. When the water content increases because of a lot of rain, the sand and water mixture becomes liquefied. When this mass of liquefied mud is on top of a hill, it will flow down under its own weight. People living at the bottom of the hill will be covered by the mud and will usually not survive. This happens in real life (also around here) and you can read about it in the paper, but you will not see in the movies. It is simply too dangerous to create a mudslide!