This underground complex includes the Dan-yr-Ogof caves, the largest cave open to visitors in Britain. The caverns themselves have been formed within layers of permeable limestone, the main base rock found in the Southern Brecon Beacons. The action of water has given rise to swallow holes (oval or circular depressions) and underground rivers. Other caves can also be found in the "waterfall country" around the village of Ystradfellte.