Ogof Ffynnon Ddu National Nature Reserve

The grassy uplands opposite Craig-y-Nos Country Park and the Dan-yr-Ogof National Showcaves Centre hide a labyrinth network of caverns and cascades, stalgamites and stalactites, passageways and potholes.

Ogof Ffynnon Ddu means Cave of the Black Spring, a reference to the vast cave system discovered beneath the moors in 1946. It’s a 300m deep underground world with an overall length of over 30 miles and the difference in level between its highest and lowest points is 308m, a record for the UK. Access to the caves is confined to experienced cavers only.

The Reserve, situated at over 350m above sea level, is one of Wales’ most exhilarating National Nature Reserves, with breathtaking views across South Wales and the Brecon Beacons. The terrain is a mixture of rocky outcrops and tufty moorland, with few obvious paths.

Herb-rich grasslands grow on the limestone, while heather and bilberry moorland covers the acid peat lying on the millstone grit. The flowers are at their best in early summer; the reserve contains rarities, including mountain everlasting, autumn gentian, mossy saxifrage and great burnet. Hairy greenweed, a low, yellow-flowering shrub, is one particularly rare species growing here.

Birds on the reserve include peregrine falcons, nightjars, ravens, ring ouzels, wheatears, red grouse and red kites. Below ground, the waters contain crustaceans that can only survive underground, and the cave is a winter habitat for moths and bats.

For a gentle, easy walk you can follow the old railway line along the side of the valley. Although this just skirts the edge of the reserve, the views are splendid. More experienced and enthusiastic walkers can access the heart of the Reserve by following paths upwards from the car park. One particularly well-defined and convenient pathway is the old tramway, a steep incline that climbs to the reserve’s eastern boundary.