300ft long drainage adit for Parys Mountain. A line of shafts follows the course of the adit. Location of dam is below Gardd Daniel Shaft.

Driven around 1850 to dewater the mines below the 45 fm level. Late in 19C, a dam was built to allow the workings to flood to the 25fm level - allowing millions of gallons of water to absorb remaining copper mineralisation. The water was released by valve into precipitation ponds at Dyffryn Adda.

via Parys Underground Group