Quicksand is real, but contrary to certain myths, quicksand is not prevalent in deserts. Quicksand can be found in damp areas such as lakes and beaches. The difference between quicksand and regular sand is the amount of water surrounding the sand particles.
In regular sand, little water surrounds the sand particles; therefore, the sand particles encounter friction when rubbing against one another. The friction provides the sand with some resistance to free movement. In quicksand, water soaks the sand so that the sand grains are not in contact with one another. This causes more fluidity and movement within the sand.
To free themselves, people should not struggle too vigorously in quicksand, because the suction force needed to pull a single leg out is tremendous, akin to the force of heaving a car upward. Quicksand, however, cannot suck a person all the way down, so quicksand cannot "swallow" people up.