Most of this opencast was worked by Parys Mine, the smaller "Hillside Opencast", being worked by The Mona Mine. The opencast represent only a small proportion of the mine as later extraction occurred through the shafts the reached depths of 300 metres, some 130 meters below sea level and now flooded.

The small lake at the bottom results from the damming of a deep level draining to the North. The water is very acidic (Sulphuric Acid - Ph2) and meant that pumps had to be made from oak instead of iron. Its orange-brown colour is due to the very high concentrations of iron (ferric - in solution). leached from oxidising sulphide minerals, as indeed is the range of yellows, reds and purple in the spoil.

The mountain gives fine vistas to the Isle of Man, Cumbria, Snowdonia and the Llyn, but abandoned shafts make the area dangerous and visitors should keep carefully to the paths.