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 Terrace and mountains of slate waste, it's about ⅓ mile in length and 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. From here, continue up the steep track alongside the Afon Cwmorthin to reach the lake. Approximately half-way along on the left is a small waterfall and pool, which is popular with local children in summer for jumping and cooling off. Tanygrisiau station on the Ffestiniog railway is just under a mile away.
Rhododendrons have now taken hold amongst the piles of slate waste. These are highly invasive and if left unchecked will eventually take over the whole area. There are plenty of small ones which are easily uprooted, so feel free to take some for the garden! The pieces of slate waste can also be put to good use in the garden - if you feel like carrying some back to your car.
