Alongside the path from Capel Curig to the summit of Moel Siabod is a flooded quarry, once used by the Moel Siabod Slate and Slab Works. The sides are mostly sheer, but it's possible to scramble down to the water in a few places. It's quite popular with local children for jumping, but always check the water below thoroughly first - before jumping in.
Parking can be found in laybys on the A5 at Grid Ref : SH736570 (Lat : 53.09519°N, 3.88911°W).