A natural arch of Carboniferous limestone about 24 metres high, carved by the sea out of a small headland. It is one of the most photographed sights in Wales, and the Natural Arch and Bridge Society calls it probably the most spectacular arch in the United Kingdom. It lost a lot of rock in Storm Ophelia in 2017 and will one day fall. A short walk away are Stack Rocks, also called the Elegug Stacks (elegug is Welsh for guillemot), limestone pillars used by nesting seabirds.
Both stand inside the Castlemartin military range, about three miles west of Bosherston, and are seen from the clifftop path. Access is normally open, but the road closes when the range is in use.