Quicksand, loose or moving fine sand mixed with water. Beneath the sand lies a bed of clay or other impervious material, which prevents the water from draining. Quicksand cannot support weight because the water keeps the sand grains apart; a person or animal stepping in it will sink. Quicksand most commonly occurs at river mouths and on flat seacoasts where sand has filled pools of water. It is difficult to recognize because it looks like ordinary sand.

The danger to hikers and other persons is relatively small, because there are comparatively few quicksand areas. Fiction writers, however, have dramatized the peril of people being swallowed into the depths. Especially vivid descriptions occur in Sir Walter Scott's Bride of Lammermoor; in Wilkie Collins's Moonstone; and in Victor Hugo's Toilers of the Sea.