Ogof Ffynnon Ddu, 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.

Penwyllt is a hamlet located in the upper Swansea Valley in Wales. It lies within a part of the Brecon Beacons National Park in the traditional county of Brecknockshire; currently administered as part of the unitary authority of Powys.