The limestone hills and valleys on the South of Senni Valley are among the most impressive caving areas in Britain.
- Dan-yr-Ogof, part of which is now a major tourist attraction, was discovered in 1912 by the Morgan Brothers of Abercraf, who penetrated the river entrance and discovered the climb up into the present show cave and reached the Third Lake. Map.
- Ogof Ffynnon Dduor Cave of the Black Spring is 308 metres (1,010 feet) deep and 48 kilometres (28 miles) long, the deepest cave in the UK and the second longest in Wales. OFD, as the cave is often called, was discovered in 1946 by Peter Harvey and Ian Nixon and this led to the formation of the South Wales Caving Club, which has its headquarters nearby. Map. The large cave system has an intricate maze-like structure and an impressive main stream passage. There are three main parts, one in the valley and two on the limestone hill above.
- Other caves and passages are known in this area, which is still being explored.