The caves of the Mendip Hills are formed by the particular geology of the Mendip Hills: large areas of limestone worn away by water makes it a national centre for caving. The hills conceal the largest underground river system in Britain.

The Mendip Hills is a range of limestone hills to the south of Bristol and Bath in Somerset, England. Running from Weston-super-Mare and the Bristol Channel in the west to the Frome valley in the east, the hills overlook the Somerset Levels to the south and the Chew Valley and other tributaries of the Avon to the north. The highest point, at 325 metres above sea level, is Beacon Batch which is the summit area atop Black Down. The hills gave their name to the former local government district of Mendip, which administered most of the local area until April 2023. The higher, western part of the hills, covering 198 km2 (76 sq mi) has been designated an Area of Outstanding Natural Beauty (AONB), which gives it a level of protection comparable to a national park.