the giant's causeway is an area of about 40,000 interlocking basalt columns. the formation is a result of an ancient volcanic eruption. it is in county antrim on the northeast coast of northern ireland. it is a world heritage site by unesco. it was named 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.