# Ogof Ffynnon Ddu

- **Country:** United Kingdom
- **Depth:** 274.5 m
- **Access:** South Wales Caving Club—see [http://www.swcc.org.uk/joomla-swcc/ website] for details
- **Length:** 59 km
- **Survey:** [http://www.clipstone1.plus.com/survey1.htm South Wales Caving Club 2014]
- **Geology:** Limestone
- **Language:** Welsh
- **Location:** Upper Swansea Valley
- **Discovery:** South Wales Caving Club 1946
- **Translation:** Cave of the black spring
- **Entrance Count:** 3

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.

![Photo of Ogof Ffynnon Ddu](https://upload.wikimedia.org/wikipedia/commons/3/39/Inside_Ogof_Ffynnon_Ddu.jpg)

**Source:** https://en.wikipedia.org/wiki/Ogof_Ffynnon_Ddu (Wikipedia, CC BY-SA)