A series of dramatic waterfalls lies between the villages of Pontneddfechan and Ystradfellte, where the Rivers Mellte, Hepste and Pyrddin pass through steep forested gorges. The finest is Sgwd-yr-Eira (Waterfall of the Snow), where you can actually walk behind the torrent. At one point the River Mellte disappears into Porth-yr-Ogof (Door to the Cave), the biggest cave entrance in Britain (3m high and 20m wide), only to reappear 100m further south.
Walks in the area are outlined in the national park's Waterfall Country publication (£1), available from visitor centres. The gentle Elidir Trail (2½ miles each way) starts from Pontneddfechan and connects with various other trails. Take special care – the footpaths can be slippery, and there are several steep, stony sections. The Four Falls walk, which takes in everything, is a hard 5.5 miles and demands proper footwear.