The limestone hills and valleys to the south of Senni Valley have some of the most impressive caving areas in Britain.
- Porth yr Ogoff at Ystradfellte map can be visited by non-cavers from a car park, or the over 1.5 miles of passages under the valley floor of the River Mellte can be explored with suitable caving clothing and equipment. The car park has toilets and some food and drink can be bought in the information Centre whch is staffed by a Brecon Beacons National Park Warden. Parties of children are often taken here on school outings. It is a good starting point for walks in 'Waterfall Country', which is the second most popular outdoor walking area in the Brecon Beacons National Park.
- The nearby Nedd Fechan caves and the silica mines near Craig-y-Ddinas can be visited by more experienced cavers.
- 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 Ddu or 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.
Updated 20/12/2018