[2] Ogof Agen Allwedd is the longest cave system on the Llangattock escarpment, with over 20.2 miles (32.5 km) of passages.
Each round trip takes upwards of six hours and some parts of the cave are prone to flooding.
This provides a bypass to an awkward rift section and that route is now blocked with a pillar.
Keys are available to people with valid permits who must sign the book on entering the cave indicating the route to be taken.
No carbide is to be used in the cave and additional permits are required for camping inside, blasting or research.