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The Giant's Causeway is an area of about 40000 interlocking basalt columns, in County Antrim on the north coast of Northern Ireland. The tops of the columns form stepping stones that lead from the cliff foot and disappear under the sea. The columns where formed by ancient volcanic activity during the Paleocene Epoch when highly fluid molten basalt intruded through chalk beds to form an extensive lava plateau. As the lava cooled, contraction occurred which caused fracturing in similar way to drying mud, with the cracks propagating down as the mass cooled, leaving the distinctive columns seen today.

The name “Giant’s Causeway” comes from the legend that the columns are the remains of a causeway built by the Irish giant Fionn mac Cumhaill (Finn MacCool), across the North Channel (Link My Secret Northern Ireland). There are identical basalt columns (a part of the same ancient lava flow) across the channel at Fingal's Cave on the island of Staffa, and it is possible that the story was influenced by this.

The site first became popular with tourists during the nineteenth century, particularly after the opening of the Giant's Causeway Tramway. The area was declared a World Heritage Site by UNESCO in 1986, and a national nature reserve in 1987 by the Department of the Environment for Northern Ireland. Much of the Giant's Causeway and Causeway Coast World Heritage Site is owned and managed by the National Trust, who took over its care in the 1960s, and it is one of the most popular tourist attractions in Northern Ireland. The remainder of the site is owned by the Crown Estate and a number of private landowners.

Link UNESCO World Heritage List
Link National Trust