The Giant’s Causeway in Antrim, Ireland is an area of about 40,000 interlocking basalt columns, the result of an ancient volcanic eruption about 50-60 million years ago. As the lava cooled, it contracted; the size of the columns is primarily determined by the speed of cooling. The tallest are about 40 ft high, & the solidified lava in the cliffs is 90 ft thick in places. Most columns are hexagonal, although there are also some with four, five, seven or eight sides. A UNESCO World Heritage site.