Large areas of limestone on the Mendips have been worn away by water, making the hills a national centre for caving. Some of the caves have been known about since the establishment of the Mendip lead mining industry in Roman times. However, many have been discovered or explored only in the 20th century. Specialist equipment and knowledge is required to visit the vast majority of the caves, but Cheddar Gorge and Wookey Hole Caves are two show caves which are easily accessible to the public. The active Mendip Caving Group and other local caving organisations organise trips and continue to discover new caverns.