Twenty minutes up the Ystradfellte lanes, the Afon Mellte river simply disappears — swallowed by Porth yr Ogof, a limestone cave mouth some 20 metres wide and 8 metres high, the largest in Wales and one of the biggest in Britain.
It’s an uncommercialised cave, not a show cave, so the pull for most visitors is the sheer scale of the entrance and the fact that the same car park is the trailhead for the famous Four Waterfalls Walk. A five-minute path leads down to the gaping mouth; the river resurfaces 300 metres downstream at a deep pool.
That resurgence pool — the “Blue Pool” — is beautiful and genuinely dangerous, so this is a place to look and photograph rather than swim. Come for the atmosphere, then walk the falls.
Worth knowing
- The cave system has 15 entrances and around 2.25 km of surveyed passage, hidden inside Carboniferous limestone.
- Afon Mellte means “flashing” or “lightning” river — it rises and falls fast with rain, sinking at the cave and resurging downstream.
- Deep inside lies the White Horse Pool, named for a natural white calcite formation shaped like a horse’s head.
- It’s a designated geosite in the Fforest Fawr UNESCO Global Geopark, and a serious caving-training venue for equipped, guided groups only.