Aerial photograph of Ogof-Yr-Esgyrn (Bone Cave), Dan-Yr-Ogof. Taken on 21 March 2007.

The caves at Dan-yr-ogof are located in the upper Swansea Valley, at the source of the Afon Llynfell, and are of carboniferous limestone formed some 315 million years ago. They were first explored in 1912 and so far some 16km of passages have been discovered.
One of three major caves at the site (the others two are the Showcave and the Cathedral cave), the Bone cave is one component of a massive system of solution caves lying beneath the Cribarth plateau, west of the Upper Tawe. Habitation and burial are well-attested in the chamber: there are hearths, animal remains and stray finds together with the skeletal remains of at least forty individuals. The latter were argued to be of Roman date, supported by associated artefacts, but this interpretation is complicated by the tight stratigraphical interdigitation of both Roman and earlier material. However, it is believed that many of the bones may be of Bronze Age origin.