Ogof Ffynnon Ddu (Welsh for Cave of the Black Spring; also known as OFD) is a cave located under a hillside in the area surrounding Penwyllt in the Upper Swansea Valley in South Wales. At 308 metres (1,010 ft) deep and 68 kilometres (42 mi) long, it is the deepest cave in the UK and the second longest in Wales.
OFD was discovered in 1946 through digging by Peter Harvey and Ian Nixon, members of the newly formed South Wales Caving Club. Major extensions were discovered in 1967. The system is famous for its intricate maze-like structure and its impressive main stream passage. It forms the Ogof Ffynnon Ddu National Nature Reserve.
One of the largest cave systems in Britain, the caves and tunnels of Ogof Ffynnon Ddu weave a long and tortuous path beneath the Tawe Valley. Deep underground, an amazing collection of specialised wildlife has developed, including cave shrimps and the pale blanched trout which haunts underground rivers. Deep cracks in the vast expanse of moorland above provide habitats to a wealth of plant life, including the lily of the valley and wood anemone.