Ogof Ffynnon Ddu
| Ogof Ffynnon Ddu | |
|---|---|
The main streamway of OFD 2. | |
Interactive map of Ogof Ffynnon Ddu | |
| Location | Upper Swansea Valley |
| Coordinates | 51°49′27″N 3°39′40″W / 51.8243°N 3.6611°W |
| Depth | 274.5 metres (901 ft) |
| Length | 59 kilometres (37 mi) |
| Discovery | South Wales Caving Club 1946 |
| Geology | Limestone |
| Entrances | 3 |
| Access | South Wales Caving Club—see website for details |
| Translation | Cave of the black spring (Welsh) |
| Cave survey | South Wales Caving Club 2014 |
| Registry | Cambrian Cave Register |
Ogof Ffynnon Ddu (Welsh for cave of the black spring), also known informally as OFD, is a cave under a hillside in the area surrounding Penwyllt in the Upper Swansea Valley in South Wales. It is the second longest cave in Wales and the deepest in the United Kingdom.