East of Reykjavik, Iceland lies the mountain, Hengill. Hengill was formed from a Palagomite tuff, it is a central volcano and has a giant magma chamber beneath it. When thick basalt lava flows cool, they form hexagonal cracks, called columnar joints. These can be found also in Ireland & Scotland.
It's just the thing that is so unbelievable you kind of want to brush it off as a hoax, but it could happen, so you're just left with this odd feeling of uncertainty. Either way it's a pretty cool thing.