Tags: Cave.
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 (1010 ft) deep and 50 kilometres (31 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.