Cynheidre was developed by the National Coal Board as one of the West Wales "super pits" alongside Abernant Colliery in the River Amman valley, a post-World War II investment intended to keep economic coal mining a viable industry in the area.
[1] Created to exploit 196 square miles (510 km2) of deep lying anthracite seams, it was envisaged to eventually encompass the working of the drift mines at the Great Mountain Colliery and Pentremawr.
Originally sunk for ventilating Glynhebog drift mine, it was extended to a depth of 760 yards (690 m) to meet the workings at Cynheidre, providing an upcast.
After these developments, Pentremawr was integrated under Cynheidre in 1973, connected both underground as well as overground by the Llanelly and Mynydd Mawr Railway.
[3] From both the structure of local geology and knowledge from workings at Pentremawr, it was known that the coal field suffered from methane gas intrusion.