Ogof Ffynnon Ddu (Welsh for Cave of the Black Spring) is a cave located under a hillside in the Upper Swansea Valley in Wales. At 308 metres (1,010 feet) deep and 48 kilometres (28 miles) long, it is the deepest cave in the UK and the second longest in Wales.

OFD (as the cave is perhaps more commonly known) was discovered in 1946 through digging by Peter Harvey and Ian Nixon. The discovery of the cave led to the formation of the South Wales Caving Club.

The system is famous for its intricate maze-like structure and its impressive main stream passage.

External link

• Virtual tour of the cave