Giant's Causeway
The Giant's Causeway is a area of 40,000 basalt columns resulting from a volcanic eruption 60 million years ago. It is the only World Heritage Site in Ireland and is located on the North Antrim Coast of Northern Ireland.
While recent scientific research1 suggests the columns were formed as a natural consequence of lava cooling, legend has it that the giant Finn McCool built the causeway to walk to Scotland without having to get his feet wet.
More on geology/formation, legends
References
1 Jagla, E. A., Rojo, A. G. Sequential fragmentation: the origin of columnar quasihexagonal patterns. Physical Review E, 65, 026203, (2002)