Manganese was discovered in the Benallt area in 1827 and was mined by small scale open-cast until 1880's. Manganese was used in the nineteenth-century to make bleach and glass, and in the twentieth-century it was used to toughen steel. A boom occurred between 1886 and 1893 which included underground exploitation. Mining was suspended between 1893 and 1900, but in 1903 the North Wales Iron and Manganese Company and the British Manganese Company were formed to develop the adjoining mines of Benallt and Rhiw respectively. The mines were finally abandoned in December 1945 and have not been worked since.

Sources:
Gwynedd Archaeological Trust
David Gwyn & Merfyn Williams (1996) `A Guide to the Industrial Archaeology of North West Wales?. Association for Industrial Archaeology