55.2408°N, 6.5114°W.
Volcanic rock formation on the northeastern coast of County Antrim, Northern Ireland, consisting of approximately 40,000 interlocking columnar basalt prisms formed by the cooling of a 50-to-60-million-year-old lava flow during the opening of the North Atlantic. Most columns are hexagonal, with some four-, five-, seven-, and eight-sided columns, formed by columnar jointing as the basalt contracted on cooling. The largest columns rise 12 metres tall; the cliff face exposes the structure across approximately 6 km of coastline. The cooling rate determined the column geometry: faster cooling on the surface produced narrower, more regular hexagons; slower cooling at depth produced thicker, irregular columns. The formation was named for an Irish folkloric giant, Fionn mac Cumhaill, who was said to have built it as a stepping-stone causeway to Scotland. Geological survey identified matching basalt columns at Fingal's Cave on Staffa, Scotland, 130 km away — the same lava flow on both sides.
Forty thousand hexagonal columns cooled into a stone carpet that crosses an ocean, and the geologists who measured it confirmed what the folktale already said.