OFD was discovered in 1946 through digging by Peter Harvey and Ian Nixon, members of the newly formed South Wales Caving Club. Exploration beyond the Boulder Chamber in 1957 revealed passages as far as the Dip Sump. Major extensions were discovered in 1967 through to Cwm Dŵr, which is now known as OFD2. The system is famous for its intricate maze-like structure and its impressive main stream passage. The cave has seen many minor incidents resulting from people getting lost or tired, or surprised by flooding, and various accidents such as falls which are handled by volunteer cave rescue teams who are alerted by the police. More prolonged rescues have included: In 1951, before cave rescue was well established, "two well-known members of the South Wales Caving Club, Lewis Railton and W.H.Little, made their way into the huge cave that bears the Welsh name of Ogof-Y Ffynnon Ddu (Cave of the Black Well or Stream) in South Wales...[when] swirling water cut off their escape...a rescue party including soldiers from Senny Bridge, men of the R.A.F., and thirty miners set to work to divert the stream above the point where it goes underground...they were brought out into the open air after being trapped for fifty-nine hours..." In 2021 the cave was the site of the longest cave rescue undertaken in Wales, after George Linnane, a 38-year-old experienced caver, fell and sustained multiple injuries. Almost 250 people were involved in the rescue, which was organised by the South and
Mid Wales Cave Rescue Team and involved manually hauling the casualty by stretcher, using relay teams of rescuers, to the top entrance. Linnane was rescued on the evening of 8 November, after 57 hours; the previous longest rescue was 41 hours. ==The cave system==