There are many known cases when people became victims of the so-called quicksand. It is easy to see something mystical in the ability of ordinary-looking sand to suddenly swallow objects on its surface, but this phenomenon has a rather simple physical explanation. The fact is that the properties of wet sand very significantly depend on the amount of water in it. Slightly moistened grains of sand easily stick together, demonstrating a sharp increase in adhesion forces, which in dry sand are determined only by surface irregularities, and therefore are small. They are made to stick together by the surface tension forces of the water films surrounding each grain of sand. In order for the grains of sand to stick well together, water should only cover the particles and their groups with a thin film, while most of the space between them should remain filled with air. If the amount of water in the sand is increased, then, as soon as the entire space between the grains of sand is filled with water, the forces of surface tension disappear and a mixture of sand and water with completely different properties is obtained. Quicksand is the most common sand, under which there is a fairly strong source of water at a depth of several meters. Most often, quicksand is found in hilly areas. Descending from the mountains, streams of water move along channels inside dolomite and limestone rocks. Somewhere downstream, water can break through a stone and rush upward in a powerful stream. If there are sandy deposits on the surface, then the flow of water coming from below will turn them into quicksand. Often the sun dries out the top layer of sand, forming a thin, hard crust on which grass can even grow. Outwardly, such a "sandy swamp" looks quite reliable and does not arouse any suspicion, but as soon as you step on it, the soil will literally float from under your feet. Although the density of quicksand is about 1.6 times that of water, swimming in quicksand is much more difficult. It is very viscous, so any attempt to move in it is met with strong opposition. The slowly flowing sand mass does not have time to fill the cavity that appears behind the displaced object, and a rarefaction, a vacuum, arises in it. The force of atmospheric pressure seeks to return the object to its original place - it seems that the sand "sucks" its prey. In addition, moving in quicksand can only be very slow and smooth, since the mixture of water and solid particles of sand is inertial with respect to rapid movements: in response to a sharp movement, it seems to solidify.