Ogof Ffynnon Ddu facts for kids

Part of the interior with a terraced cascade and three standing and climbing cave explorers wearing safety equipment

Ogof Ffynnon Ddu (which means cave of the black spring in Welsh), often called OFD for short, is a huge cave system. It is hidden under a hillside near Penwyllt in the Upper Swansea Valley in South Wales. This amazing cave is the second longest in Wales and the deepest in the whole UK.

Exploring Ogof Ffynnon Ddu

This incredible cave was first found in 1946. Two members of the new South Wales Caving Club, Peter Harvey and Ian Nixon, discovered it by digging. They kept exploring and in 1957, they found more passages, reaching a spot called the Dip Sump.

Even bigger parts of the cave were discovered in 1967. These new sections led to an area called Cwm Dŵr, which is now known as OFD2. The cave system is famous for its many twisting paths, like a giant maze, and its impressive main stream passage.

Today, Ogof Ffynnon Ddu is part of a special area called a National Nature Reserve. This reserve also includes the old buildings of a brick factory, with its kilns and old tram tracks. The houses where the factory workers used to live are now used by the South Wales Caving Club.

Inside the Cave System

Ogof Ffynnon Ddu is incredibly deep, going down about 274 meters (900 feet). It stretches for at least 50 kilometers (31 miles) underground. This makes it the deepest cave in the UK and the second longest in Wales. It is one of the biggest cave systems in Britain.

The passages and chambers of Ogof Ffynnon Ddu wind their way under the east side of the Tawe Valley. A stream flows through the cave, cutting through dark Limestone rock. This creates beautiful waterfalls, fast-flowing rapids, deep holes, and smooth, wavy walls. The cave system is split into three main parts: OFDI (the lower, western part), OFDII (the middle section), and OFDIII (the upper, eastern part).

Underground Wildlife

Special kinds of animals live in the cave's dark environment. You might find cave shrimps and a unique type of pale trout. These trout are found only in very pure underground freshwater with enough tiny living things (plankton) for them to eat.

Above ground, on the rocky moorland, deep cracks provide homes for different plants. These include the lily of the valley and wood anemone. You can also see mossy saxifrage (Saxifraga hypnoides) growing on the limestone rocks.

A Special Nature Reserve

The name Ogof Ffynnon Ddu also refers to a large area of moorland, rocks, and cave systems. The famous cave is just one part of it. This area was made a National Nature Reserve (NNR) because it has unique geology (how the rocks are formed), many different kinds of plants, an interesting industrial past, and special animals and plants living underground.