Brecon Beacons, Wales
A karstic geosite in Fforest Fawr Geopark & Brecon Beacons National Park
'Porth yr Ogof is arguably Wales' largest cave entrance.
Here the Afon Mellte embarks on a subterranean journey for the next 1/4 mile through the Carboniferous Limestone belt deep in the heart of the Fforest Fawr Geopark and Brecon Beacons National Park.'
From Alan Bowring
“Waterfall Country” is one of the most visited areas in the Brecon Beacons National Park. Here you’ll find a number of rivers draining the Old Red Sandstone hills further north flow south towards the Vale of Neath through steep wooded gorges cut into rocks of Carboniferous age. Just east of the village of Ystradfellte, one of these rivers, the Afon Mellte, flows into surface exposures of the southerly tilted Carboniferous Limestone and embarks on an underground journey at Church Sink.
From this site the riverbed is usually dry for half a mile downstream. However, in wetter weather, with the river in flood, large volumes of water flow south and enter the imposing entrance to a maze-like cave system created by the erosion and solution of Carboniferous Limestone. At 20m wide and 3 m high Porth yr Ogof is the largest cave entrance in Wales.
The river emerges 300m to the south at a resurgence popularly known as the Blue Pool. This site can be reached on foot by following the long-abandoned surface route of the river down-valley from the National Park Authority-managed car park and visitor facility at Cwm Porth.
Visitors can make their way carefully down from the car park to stand in the main cave entrance. This is the haunt of innumerable education and adventure groups set on exploring the cave passages beyond the huge bedding plane roofed entrance.
But beware! The further reaches of the cave are the preserve of experienced cavers only – its hazardous nature has given rise to a number of fatalities over the years so it’s wise to exercise great caution in this area. 
If you want to explore further, get in touch with one of the local adventure activities providers.
There are fourteen other known entrances to the Porth yr Ogof system, some of which are to be found in the long-dry gorge above the cave, now festooned with ferns and like much of the area protected as a Special Area of Conservation (SAC) and Site of Special Scientific Interest (SSSI). The magical character of the area has also led to it being sought out as a film location - the BBC’s ‘Merlin’ TV series being a recent one.
Information for Visitors
The car park and toilets are open throughout the year and the warden’s hut and shop are staffed most days. Fforest Fawr Geopark has installed interpretive panels at this site.
At nearby Pontneddfechan is the Waterfalls Centre with interpretive displays on this locality and other geological and cultural attractions of the Geopark.
Getting there
The Waterfalls Centre in Pontneddfechan makes a good start for any visit. Follow signs off the A465 ‘Heads of the Valleys Road’ for ‘Waterfalls Information Centre’ (OS grid ref SN 901076, postcode SA11 5NR). Continue towards Ystradfellte for the pay & display car park at Porth yr Ogof (OS grid ref SN 928124, postcode CF44 9JF) From Brecon, follow signs for ‘Ystradfellte’ off the A4059 road north of Penderyn.
Text courtesy of Alan Bowring, Fforest Fawr Geopark