The increase is due to the formation of sand sediment, which has a very high viscosity. It's the difficulty of moving this dense sand that causes the problem.
Water has to be introduced into the sand sediment to loosen it, and this requires considerable amounts of force. The authors estimate that the force needed for someone to pull their foot out of quicksand at a speed of a centimeter a second would be the equivalent of that required to lift a medium-size car.
What to Do When You're Stuck
If you do step into quicksand, says study co-author Daniel Bonn, you'll only sink in a little deeper than your waist. "I would say there would be some pressure on the chest, but not enough to cause serious trouble."
So how do you get out? Don't ask your friends to tug on you; they're likely to pull you "into two pieces if [they] try hard to pull [you] out," said Bonn, a physics professor at the Van der Waals-Zeeman Institute at the University of Amsterdam.
"The way to do it is to wriggle your legs around. This creates a space between the legs and the quicksand through which water can flow down to dilate [loosen] the sand," he explained. "You can get out using this technique, if you do it slowly and progressively."
Origins of the Myth
A person will gradually begin to sink in quicksand, and movement will make the victim sink faster. This may be the origin of the advice to "never struggle if you're caught in quicksand."
But no amount of struggling will send you in over your head. Bonn suggests that it isn't struggling that can get you into trouble, but getting caught in quicksand near the sea, which is generally where quicksand is found.
When the high tide comes in, you could drown.
Free E-Mail News Updates
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).
SOURCES AND RELATED WEB SITES