The Giant's Causeway lies at the foot of the basalt cliffs along the sea coast on the edge of the Antrim plateau in Northern Ireland. It is made up of some 40,000 massive black basalt columns sticking out of the sea. The dramatic sight has inspired legends of giants striding over the sea to Scotland. Geological studies of these formations over the last 300 years have greatly contributed to the development of the earth sciences, and show that this striking landscape was caused by volcanic activity during the Tertiary, some 50–60 million years ago.

Źródło: World Heritage Convention
http://whc.unesco.org/en/list/369