This Bronze Age platform cairn was originally sited further down the valley (SN75108690), and was flooded when the Nant y Moch Reservoir was created in 1961. The cairn was rediscovered in 1984 when the level of the reservoir was low. After excavation it was moved piece by piece to its current location (you can still make out some of the numbers painted on the stones to identify them).
It's usually pretty easy to find the cairn, but in poor weather you might not spot it from the road because its surrounded by reeds. Coming from Ponterwyd you need to cross the Nant y Moch Dam itself, then after the first cattle grid start looking out for the site on your right. There is a stock fence around the cairn (but no gate to stop the sheep or visitors from getting in). There's also a conveniently placed lay-by for parking in.