Llyn Y Foel is a shallow lake about 1750 feet above sea level, lying to the south of Moel Siabod and the path to the summit. Unfortunately, it has plenty of water weeds, and due to the shallow water it's not the best lake for swimming, but nevertheless a useful spot to cool off on a hot day.
Parking can be found in laybys on the A5 at Grid Ref : SH736570 (Lat : 53.09519°N, 3.88911°W).