The natural formation in Northern Ireland made of hexagonal basalt columns and recognised as a World Heritage Site is the Giant's Causeway.
Located on County Antrim's north coast, the Giant's Causeway contains about 40,000 tightly packed basalt columns. They formed roughly 60 million years ago during intense volcanic activity, when cooling lava contracted and fractured into the distinctive polygonal shapes. Most columns are hexagonal, although four-, five-, seven- and eight-sided examples also occur.
Irish legend gives the site a different origin. The giant Fionn mac Cumhaill, or Finn McCool, supposedly built a causeway to Scotland to confront a rival giant. The scientific explanation is less dramatic but equally remarkable: the columns are the cooling geometry of a vast ancient lava field.
UNESCO inscribed the Giant's Causeway and Causeway Coast in 1986 for its exceptional natural beauty and geological importance. The tallest columns reach about 12 metres, and some formations have acquired names such as the Giant's Boot and the Organ. Stonehenge is a prehistoric monument in England, not a basalt formation.