Ogof Ffynnon Ddu Cave
At 308m, Ogof Ffynnon Ddu in the Upper Swansea Valley is Britain’s deepest cave.
The Byfre Fechan stream sinks at Pwll Byfre (SN 875166) and emerges at Ffynnon Ddu (SN 846152), near Craig-y-Nos Country Park in the southwestern section of the Brecon Beacons National Park. This cave is both Britain’s deepest and also Britain’s third longest with around 30 miles of passages.
Walkers following the Beacons Way across the Ogof Ffynnon Ddu National Nature Reserve are more or less tracing the line of the cave deep beneath them. Numerous small caves and shakeholes (natural depressions in the limestone pavement) can be seen in this area.
Click here to find out more about the Ogof Ffynnon Ddu National Nature Reserve.