Photo of Porth yr Ogof, a cave near Ystradfellte in Bannau Brycheiniog, the Powys, mid Wales.

© Alan Hughes / Geograph · CC BY-SA 2.0

Porth yr Ogof

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Porth yr Ogof has the largest cave entrance in Wales, a gaping mouth some 20m wide where the Afon Mellte disappears underground near Ystradfellte. You can walk into the entrance chamber and follow the river down to where it resurges. Take real care: the resurgence pool has drowned several people, and the rock is slippery. Best treated as a spot to look at, not to wade or swim.

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