A swallow hole, also known as a sinkhole, is a natural depression in the ground caused by the collapse of a cave roof. It is often associated with karst topography and can be found in areas with limestone or other soluble rocks. The term 'swallow hole' comes from the fact that these depressions often fill with water, creating a pool that swallows up anything that falls into it. The crossword solution for the term 'swallow hole' is NEST.