Swallet is a term used to describe a small hole or depression in the ground that leads to an underground cave system. Synonyms for swallet include sinkhole, pothole, doline, swallow hole, cenote, and karst. Sinkhole is the most commonly used term and describes a depression in the ground caused by the collapse of a surface layer into an underground void. Pothole is a similar term used in the UK to describe a natural depression or hollow in a limestone or chalk surface. Doline is a term commonly used in Europe to describe a closed depression in a karst landscape. Swallow hole and cenote are used to describe a vertical shaft or hole in the ground that leads to an underground river or cave system. Karst is a term used to describe a region of exposed limestone with complex underground drainage systems.