Dan-Yr-Ogof Showcaves

Dan yr Ogof, at the National Showcaves Centre for Wales, is a 17-kilometre long cave system in south Wales, about 5 miles north of Ystradgynlais and 15 miles southwest of Brecon, in the Brecon Beacons National Park. It is the main feature of a show cave complex, which is claimed to be the largest in the United Kingdom and is a tourist attraction in Wales. The first section of the cave system is open to the public, but the extensive cave system beyond is scheduled as a national nature reserve and is open only to bona fide cavers.The bones of some 42 humans, as well as numerous animal bones, have been found in one of the nearby chambers of this cave system. In a 2005 poll of Radio Times readers, Dan yr Ogof was named as Britain's greatest natural wonder.

Source: Wikipedia