The Giant’s Causeway is a tightly packed cluster of some 40,000 columns of basalt (a black volcanic rock). It is located at the foot of a sea cliff that rises 300 ft (90 m) on the northern coast of Northern Ireland. Although legend says the formation was created by a giant named Finn McCool, it in fact resulted from a volcanic eruption some 60 million years ago, one of a series that brought about the opening up of the North Atlantic. The eruption spewed up vast amounts of liquid basalt lava, which cooled to form the columns. They are up to 42 ft (13 m) tall and are mainly hexagonal, although some have four, five, seven, or eight sides.