Thanks to Chris Ford for recommending these excellent Welsh caves – Deskarati –
Dan yr Ogof is a 17-kilometre (11 mi) long cave system in south Wales, five miles north of Ystradgynlais and fifteen miles south west of Brecon within the Brecon Beacons National Park. It is the main feature of a show cave complex (advertised as The National Showcaves Centre for Wales) which is claimed to be the largest in Europe and is one of the major tourist attractions 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 the greatest natural wonder in Britain.
The cave was first explored by two local brothers in 1912, Tommy and Jeff Morgan, using candles and primitive equipment. Completely unsure of what they would discover, they armed themselves with a revolver. The Morgan brothers’ initial expedition was halted at a large lake, which they later managed to cross by coracle. They eventually crossed three more lakes in the same manner, but were stopped by a tight crawl. This squeeze, known as the Long Crawl, was first passed by Eileen Davies, a member of Swansea University Caving Club in 1963, although it is claimed that it was first passed by Peter Ogden of the same Club in the October before. The initials ‘PO’ were found by Eileen Davies at the pitch to Gerard Platten Hall. Peter Ogden had not descended the pitch due to a period of bad weather. Exploration has been steadily continued by later cavers who have extended the cave to its present 11 mile length. Some of this length was reached by cave diving. One of these explorers was Martyn Farr, who wrote a book about the system in which he claims that the system will eventually be extended to at least 150 kilometres (90 mi).
via Dan yr Ogof