From Wikipedia: http://en.wikipedia.org/wiki/Giants_causeway
The Giant's Causeway (known as Clochán an Aifir or Clochán na bhFomhórach in Irish and tha Giant's Causey in Ulster-Scots) is an area of about 40,000 interlocking basalt columns, the result of an ancient volcanic eruption. It is located in County Antrim on the northeast coast of Northern Ireland, about three miles (4.8 km) northeast of the town of Bushmills. It 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. In a 2005 poll of Radio Times readers, the Giant's Causeway was named as the fourth greatest natural wonder in the United Kingdom. The tops of the columns form stepping stones that lead from the cliff foot and disappear under the sea. Most of the columns are hexagonal, although there are also some with four, five, seven or eight sides. The tallest are about 12 metres (39 ft) high, and the solidified lava in the cliffs is 28 metres thick in places.
The Giant's Causeway is today owned and managed by the National Trust and it is the most popular tourist attraction in Northern Ireland.