The giants causeway is the result of an ancient volcanic eruption around 50 to 60 million years ago. It is an area of about 40,000 interlocking basalt columns. Most of the columns are hexagonal.
It is located on the coast of County Antrim, near to the town of Bushmills home of the famous Irish Whiskey.
The Department of the environment of Northern Ireland declared it a nature reserve in 1987. It was declared a world heritage site by UNESCO in 1986.
According to legend, the columns are the remains of a causeway built by a giant. The story goes that the Irish giant Fionn mac Cumhaill (Finn MacCool), from the Fenian Cycle of Gaelic mythology, was challenged to a fight by the Scottish giant Benandonner.
Fionn accepted the challenge and built the causeway across the North Channel so that the two giants could meet.
In one version of the story, Fionn defeats Benandonner. In another, Fionn hides from Benandonner when he realises that his foe is much bigger than he. Fionn's wife, Oonagh, disguises Fionn as a baby and tucks him in a cradle. When Benandonner sees the size of the 'baby', he reckons that its father, Fionn, must be a giant among giants. He flees back to Scotland in fright, destroying the causeway behind him so that Fionn could not follow.
Across the sea, there are identical basalt columns (a part of the same ancient lava flow) at Fingal's Cave on the Scottish isle of Staffa, and it is possible that the story was influenced by this.