My friend Stephanie reminded me a few weeks ago that I haven't yet visited Ponyhenge, a spontaneous collection of rocking horses, plastic ponies, and other horse toys in a field by the side of the road in Lincoln, Massachusetts (half an hour from Boston). Since 2010, the figures have been mysteriously appearing, and multiplying; they're occasionally rearranged into a circle, or a racing formation, or left to total chaos. Nobody knows how Ponyhenge started, though Atlas Obscura has posited some theories. I plan to check it out this summer.