Llyn Cwmorthin lies just above the village of Tanygrisiau (pronounced 'tanner-grisher') near Blaenau Ffestiniog (pronounced 'Bly-ner fess-tinny-og'). Overlooked by the derelict cottages of Cwmorthin Uchaf and mountains of slate waste, it's a good place for a swim on a hot day. Although quite high up, there is some algae growth on the slate waste underwater (some of which can have sharp edges), so it's best to enter the water by sitting down and easing yourself in, rather than trying to walk over the lumps of slate.
A small car parking area can be found at Grid Ref : SH682454, but the lane leading up to it is very steep and narrow. Tanygrisiau station on the Ffestiniog railway is just under a mile away. There's plenty to explore here after a swim in the lake, and the pieces of slate waste can be put to good use in the garden - if you feel like carrying some back to your car.