| 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)[1] |
| Length | 59 kilometres (37 mi)[2] |
| 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 | Survey |
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.
- ^ "Ogof Ffynnon Ddu - Surveys". South Wales Caving Club. Retrieved 21 September 2015.
- ^ "Ogof Ffynnon Ddu 1". Cambrian Cave Registry. Cambrian Caving Council. Retrieved 4 November 2019.