IT IS a staple scene of B-movies and Westerns: the cowboy stumbles into a patch of quicksand and is sucked under until only his stetson remains on top, or sinks up to his neck until hauled out by his sidekick.

Both scenarios have now been proved to fly in the face of physics. Research has shown that it is impossible for people to sink into quicksand much beyond the waist — but it is equally impossible to pull someone out once they are stuck.

Any attempt to drag a person out with a horse or truck would put them in much greater danger than leaving them be: the forces involved would tear them apart. To pull a person’s foot out would require as much force as it takes to lift a family car, and the body would give way before the sand relinquished its grip.

At least one commonly held belief about quicksand, however, does turn out to be true, the first detailed analysis of the physics of the treacherous goo has found. Struggling and thrashing your legs will make you sink more quickly. It is movement that makes unstable the mixture of sand, clay and salt water that makes up quicksand.

The research, led by Daniel Bonn, of the Ecole Normal Supérieure in Paris, gives the lie to dozens of famous film scenes in which the protagonists drown or are rescued.

Perhaps the most famous example is the 1959 Hammer film The Hound of the Baskervilles, in which a woman is sucked to her death by quicksand and Sherlock Holmes, played by Peter Cushing, narrowly escapes the same fate.

Other films to feature implausible quicksand scenes include Lawrence of Arabia, Flash Gordon, King Solomon’s Mines, The Jungle Book, Ice Cold in Alex and Crusoe.

In the study, Dr Bonn recreated quicksand in the laboratory to show how it becomes unstable, how far objects of different densities will sink into it and how anything stuck in it can be removed. The results are published today in Nature.

He was inspired by a holiday to Iran with his wife, on which they passed a field of quicksand. “The local shepherds told me that complete live camels had disappeared in it, and I told them that couldn’t be true,” Dr Bonn said. “When I got back, I decided to prove it.”

Quicksand is a mixture of fine sand, clay and salt water, in which the grains are delicately balanced in a very unstable matrix. This makes it superficially solid, but it collapses into a gunge when disturbed. Dr Bonn likened it to the game pick-up sticks, in which sticks have to be removed from a pile without disturbing it.

Once the quicksand is disturbed by movement on the surface, it liquefies, causing anything on top to sink into it very easily. The sand and clay then fall to the bottom of the mixture, creating a thick sediment that ultimately prevents further sinking, but makes it very difficult to escape.

Just how far anything will sink depends on its density, and how much it moves. “Any unfortunate victim should sink halfway into the quicksand, but could then take solace from the knowledge that there would be no risk of being sucked beneath the surface,” Dr Bonn said. One option to get out is for the person to wiggle his feet gently, gradually allowing water in around the edges. He could then pull himself up by fractions of an inch at a time.

It is only possible to drown by falling in head first. “This I think is where the myth of drowning in quicksand comes from,” Dr Bonn said.

Bad film physics

Broken windows: hero jumps through window, not a scratch. In reality jumping through plate glass would slice off body parts.

Flashing bullets: sparks as bullets ricochet off walls, doors etc. Bullets are made of copper-clad lead or lead alloys which do not produce sparks in this way.

Bangs in space: sound is carried by a wave that needs matter to propagate. Outer space is, in essence, a vacuum in which no one can hear you scream.

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