On the hills to the south of Fairbourne beach are the remains of old slate quarries, one of which is now flooded and known as the Blue Lake. Although it's on private land, it's a popular swimming spot, and can be accessed through a short tunnel. To find it, take the coastal path (heading south) along Fordd Panteinion and follow the path to the quarry workings. Some serious accidents have occurred here, so don't swim if you're not used to cold, deep water and (unless you're a complete idiot) don't jump off the cliffs.
Unfortunately, due to a vile, inconsiderate minority of litter-louts who leave their mess behind, the landowner has now blocked access to this lake.