Porth yr Ogof is a cave located near the village of Ystradfellte, near the southern boundary of the Brecon Beacons National Park in Wales. In 1998 the cave's passageways had been measured as over 2.25 km in length. Among the cave's fifteen entrances is the largest cave entrance in Wales and one of the largest in the UK standing at nearly 20m wide and 8m tall (but many of those entrances are only accessible to cave divers).