Some of the longest and most tortuous cave systems are found in the east of the Brecon Beacons National Park.
Access to the major cave systems of Agen Allwedd and Ogof Daren Cilau beneath Mynydd Llangatwg is gained through entrances in the imposing Llangattock Escarpment across the Usk Valley from Crickhowell.
Both drain ultimately into the Clydach Gorge, their combined resurgence being a deep pool known as Pwll-y-Cwm and of interest to the general visitor as well as the caver.
A third major system, Ogof Craig-a-Ffynnon, can only be entered from its resurgence in the Clydach Gorge. Numerous other smaller caves have entrances both in the escarpment and the gorge.