The Giant's Causeway is an area of about 40,000 interlocking basalt columns resulting from a volcanic eruption. It is located on the North East coast of Northern Ireland, about 2 miles north of the town of Bushmills. It was declared a World Heritage Site by UNESCO in 1986, and a National Nature Reserve in 1987. 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 Giant's Causeway is owned and managed by the National Trust. 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, however there are some with four, five, seven and eight sides. The tallest are about 12 metres (36 ft) high, and the solidified lava in the cliffs is 28 metres thick in places.