Funnel-shaped hollow in an area of limestone. A sink hole is usually formed by the enlargement of a joint, or crack, by carbonation (the dissolving effect of water). It should not be confused with a swallow hole, or swallet, which is the opening through which a stream disappears underground when it passes onto limestone.
A depression in the ground communicating with a subterranean passage (especially in limestone) and formed by solution or by collapse of a cavern roof; SYN. swallow hole.