The Giant's Causeway is the most popular tourist attraction in Northern Ireland. The Giant's Causeway is an area of about 40,000 interlocking basalt columns, the result of an ancient volcanic eruption. It was declared a World Heritage Site by UNESCO in 1986. 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 high, and the solidified lava in the cliffs is 28 metres thick in places.