The deep, fine sand we call quicksand forms on sand flats at the shore or at the bottom of streams or rivers which flow on top of bases of clay.
The smooth, rounded grains of sand slide past each other in wavy movements called swells. This movement is created because the water has no place to drain on the clay base. It cannot penetrate the clay, so the sand stays saturated with water.
The sand and water mixture is like a thick fluid that swallows anything heavy that moves into it. But if a person is caught in quicksand, he should lie flat on his back with his arms outstretched. This will keep him afloat until he can roll out of it onto dry land.
In 1875, an entire train sank into quicksand in Colorado and was never found again, even though probes were sent 50 feet deep!