The Blue Lagoon sits just behind the beach, a flooded slate pit twenty-five metres deep and a colour that isn't quite blue and isn't quite green. It got that way because local fishermen blasted a channel through the quarry wall to let the sea in, turning an abandoned pit into a sheltered harbour. The colour, for what it's worth, comes from the mineral content of the slate rather than the depth. In 2012 the Red Bull Cliff Diving World Series turned up and had people leaping off a 27-metre platform into it, which they did again in 2013 and 2016.
That is the whole of Abereiddy, more or less. It is a hamlet, not a village. There is no pub, no shop, no church — a small pebble-and-black-sand beach, a car park, and the roofless remains of a quarry village up on the clifftop. In summer a refreshments van parks by the beach and the public toilets open from Easter to October half term. For anything else you drive.
The ruins are worth the walk up. Slate was worked here through the 19th century by the St Brides Slate Company, and a tramway opened in 1851 to carry it to the harbour at Porthgain. When the quarry closed around 1910 the workers stayed on in their cottages — The Row, The Street — until storms and a hard winter around 1920 wrecked the place and emptied it. A 1938 storm surge finished off the shoreline cottages, residents wading out through the water. The foreman's house and the powder store still stand on the clifftop, roofless, above the flooded pit.
The coast path from Abereiddy to Porthgain is about 1.9 miles each way, or a circular of around four. Visit Pembrokeshire calls it "one of the best stretches along the entire coast path," and for once the tourist board is not overselling — two harbours, two beaches, a National Trust stretch and the quarry ruins in a single walk, much of it following the old 1851 tramway line. Halfway along is Traeth Llyfn, a sandy cove reached by steep metal steps, with a strong undertow and a habit of getting cut off at high tide.
Porthgain, a mile off, is where you eat. The Sloop Inn has been at the head of the harbour since 1743 and does 10oz ribeye and sirloin steaks, an "Angry Dog" burger, fresh Porthgain crab specials and a ham ploughman's; it's open for food every day of the year except Christmas Day, dogs welcome, with harbour-side seating. Next door the Shed Bistro does fish and chips and seafood in a former machine shop — the lobster and crab landed by Rob, the owner, and his son Jack. Two miles the other way, Trefin has a pub by Aberfelin beach.
The nearest station is Haverfordwest, fifteen miles off; you reach Abereiddy down narrow lanes off the A487, turning at Croesgoch where the sign points to the beach and, for reasons of its own, the Pembrokeshire Sheepdogs.
The circular clifftop watchtower on Trwyn Castell, built around 1800, has been called a lookout, a navigation marker, and a "tea pagoda" for the quarry managers' wives, depending on who you ask.