It’s quicksand. Quicksand is much thicker than water, which is why things sink so slowly. By that same logic, one can walk on its surface if you go fast enough and with little enough pressure, like skipping a stone on water.
CoolScienceGifs: Quicksand: Another non-newtonian fluid. You could, if the fancy took you, walk on a vat of cornflour solution in the same way.
Quicksand is a colloid hydrogel consisting of fine granular material (such as sand or silt), clay, and water.
Quicksand forms in very wet, saturated loose sand when the sand is suddenly agitated and moved. When water in the sand cannot escape, it creates a liquefied soil that loses strength and cannot support weight. Quicksand can form in standing water or in upwards flowing water (like a spring). A minor (less than 1%) change in the stress on the quicksand will cause a sudden decrease in its viscosity.
The saturated sand may appear quite solid until a sudden change in pressure or shock initiates liquefaction. This causes the sand to form a suspension and lose strength. The man stepping on the sand above is initiating a sudden change in pressure, a repeated shock, and it’s causing the sand to become fluid-like and less able to hold his weight. The cushioning of water gives quicksand, and other liquefied sediments, a spongy, fluid-like texture.
After an initial disturbance — such as a person attempting to walk on it — the water and sand in the quicksand separate and hard bits of sand form; it is because of these harder areas of sand that the ‘runnyness’ of the quicksand seems to decrease suddenly. Then, someone stepping on it will start to sink.
In this GIF, the person is moving so fast, that they are suddenly separating the sand and water with each step with the added pressure of their weight- making it momentarily harder and more stable to stand on. If they stayed still for any length of time, they would sink. I live next to the sea, and I know that if you stamp up and down enough on wet sand like this, you encourage the sand particles to liquefy. Try it!
Things that end up in quicksand, e.g. people/animals/Indiana Jones, will sink down, but only to the level of the weight of the quicksand they have displaced. Therefore quicksand is not as dangerous as it’s made out to be in the movies- you’re more likely to float. This is because sand and water together are a lot heavier than you- the solution has a higher density. You only have to displace a little of the quicksand to float so that it matches your weight.
Continued or panicked movement may cause a victim to sink deeper, leading to belief that quicksand is dangerous. Because it increasingly impairs human movement, it allows harsher elements such as sunlight, dehydration, carnivores, omnivores, hypothermia or tides to harm a trapped person.
Quicksand may be escaped by slow movement of the legs in order to reduce viscosity of the fluid, and rotation of the body so as to float flat on your back.