Marchlyn Mawr reservoir is a high level lake in Snowdonia behind Elidir Fawr mountain.

It is used as the high level water source for Dinorwig power station, a closed-loop pumped storage hydroelectric generating facility.

It is said that nearby lies the cave containing Arthur's treasure,[citation needed] a source of bedazzlement to the wanderer who sees it, and of disaster to the pilferer who touches it.

The lake itself lies in between two mountains Carnedd y Fillast and Elidir Fawr and lies at a height of 636m above sea level.

From the reservoir, a 10.5-metre (34 ft) diameter tunnel runs for 1.7 km (1.1 miles) to a 10 m (33 ft) diameter vertical shaft.