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 to 1979). The reservoir holds about 9.2 e6m3 of water. From the reservoir, a 10.5 m diameter tunnel runs for 1.7 km to a 10 m diameter vertical shaft. From the bottom of the shaft, a 9.45 m diameter tunnel leads to the power station, 670 m away. It is possible to cycle up the road to the reservoir but a gate at the bottom prevents cars from driving on the road.