Death-by-quicksand is a favourite of B-movie directors. But would a hapless cowboy or bandit really be sucked under? BBC Future pokes a tentative toe into the science…

We’ve all seen the films.  A man is caught in quicksand, begging onlookers for help, but the more he struggles, the further down into the sand he is sucked until eventually he disappears. All that’s left is sinister sand, and maybe his hat. There are so many films featuring death by quicksand that Slate journalist Daniel Engbar has even tracked the peak quicksand years in film. In the 1960s, one in 35 films featured quicksands. They were in everything from Lawrence of Arabia to The Monkees.

Yet the evidence that the more you struggle, the further you sink until you drown, is rather lacking. Quicksand usually consists of sand or clay and salt that’s become waterlogged, often in river deltas. The ground looks solid, but when you step on it the sand begins to liquefy. But then the water and sand separate, leaving a layer of densely packed wet sand which can trap it. The friction between the sand particles is much-reduced, meaning it can’t support your weight anymore and at first you do sink. It is true that struggling can make you sink in further, but would you actually sink far enough to drown?

Daniel Bonn from the University of Amsterdam was in Iran when he saw signs by a lake warning visitors of the dangers of quicksand. He took a small sample back to his lab, analysed the proportions of clay, salt water and sand, and then recreated quicksand for his experiment. Instead of people, he used aluminium beads which have the same density as a human. He put them on top of the sand and then, to simulate the flailing of a panicking human, he shook the whole model and waited to see what happened. Would the aluminium beads “drown”?

The answer was no. At first they sunk a little, but as the sand gradually began to mix with water again, the buoyancy of the mixture increases and they floated back up to the top. Bonn and his team tried placing all sorts of objects on his lab-made quicksand. If they were of density equivalent to a human they did sink, but never completely, only half way.   

Although quicksand doesn’t continue to pull you right under, if you can’t get free in time, a high tide can sweep across you

Why then, if physics predicts that you don’t endlessly sink further and further down, are there occasional tragic accidents where people do die, such as a mother of two who drowned in 2012 while on holiday in Antigua?

The reason is that although quicksand doesn’t continue to pull you right under, if you can’t get free in time, a high tide can sweep across you. This is really when quicksand can be dangerous.

So struggling alone won’t drown you, but we do still need to be wary. If you want to free yourself without waiting for rescue or for the sand to liquefy again, then Bonn’s research showed that just to release one foot, you would need to provide a force of 100,000 newtons – the equivalent of the strength to lift a medium-sized car.

In the lab Bonn’s team found that salt was an essential ingredient because it increased the instability of quicksand, leading to the formation of these dangerous areas of thick sediment. But then another team, this time from Switzerland and Brazil, discovered a kind of quicksand that doesn’t need salt. They tested samples from the shores of a lagoon in north eastern Brazil. They found that bacteria formed a crust on the top of the soil, giving the impression of a stable surface, but when stepped on the surface collapsed. But even then the good news is that basins formed from this kind of soil are very rarely deeper than the height of a human, so even if someone did slip into the quicksand they wouldn’t drown.  

Dry quicksand, however, is another matter entirely. The quicksand effect means that falling into a silo full of grain can often be fatal.

To survive a fall into dry quicksand, you need outside help as quickly as possible

In 2002 a case report was published telling the tale of a man who fell into a grain store late one evening on a farm in Germany. By the time the firefighters were able to establish which of eight tanks he was in, the grain was up to his armpits and acting according to the classic idea of quicksand, was dragging him down. Each time he exhaled, the volume of his chest reduced, causing grain to rush to fill the gap and making it progressively harder for him to breathe.

A doctor was lowered down on a rope to give him oxygen and a harness was placed around the man’s chest. But soon he was experiencing agonising chest pain and the doctor developed an asthma attack brought on by the dust. The firefighters did come up with a clever solution, though. They lowered a cylinder over the man’s body. Then as they sucked the grain out with an industrial vacuum, the grain couldn’t fall more tightly around him, and he survived.  

To survive a fall into dry quicksand, you need outside help as quickly as possible, but what if you find yourself in some wet quicksand, not drowning, but stuck? You need to wiggle your leg a little in order to introduce water to the sand around your feet to liquefy the sand again. The idea is to stay calm (which might be easier said than done), lean back and spread out to spread your weight more evenly and wait until you float back up to the surface. And don’t forget your hat.

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