From mental_floss:

The Giant’s Causeway is on the northeast coast of Northern Ireland. Legend says that the giant Finn McCoolbuilt the causeway so he could fight his enemy Benandonner in Scotland. The rock formation looks like a set of mostly hexagonal man made stepping stones, but this is a natural formation of basalt laid down by volcanic activity. During the Tertiary period some 65 million years ago, this piece of land was near the equator. Lava tubes pressed up through a chalk layer to form the pillars. The geometric shapes were caused by crystallization of the basalt as it cooled and cracked. The causeway is open to the public and can be reached by a shuttle bus.Devil’s Postpile is a similar formation in California.

Image from jonmcalister.