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, 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 and the reservoir took four years to build (from 1975-1979). The reservoir holds about 9.2 million cubic metres (320 million cubic feet) of water. 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. From the bottom of the shaft, a 9.45 m (31 ft) diameter tunnel leads to the power station, 670 metres (2,200 ft) away.