Slate was quarried at this site on the water's edge in Abereiddi right up until 1910 and then transported by tramway to the harbour at Porthgain. After the mining stopped, a channel was blasted through to the sea, flooding the pit and creating a brilliantly blue-green pool surrounded by a bowl of sheer stone walls. You can certainly swim here, but be aware that the water is very deep and commensurately cold.
From Porthgain, the Blue Lagoon is best reached via a spectacular 30-minute walk west along the coast path, past the ruins of workers' cottages and quarry buildings.