I've never been. But it's certainly on the Bucket List, and I'm sure I'm not the only BHGPer to have it on their list: The Giant's Causeway. The crazy columns below are all basalt from an ancient volcanic eruption, though nobody knows the physics of their formation nor the reason they're mostly hexagonal but also have some 4- 5- or 8-sided columns peppered in. The ~40,000 of these columns form the Giant's Causeway and the crazy pictures of cliffs, hills, valleys, and caves that you see below. Many more on GIS if these give you the same travel lust as they do me. Happy Hamsterdam!