The Atomium was built for Expo 58, the World’s Fair in Brussels, Belgium in 1958. It is a building in the shape of a single unit cell of an iron crystal, or nine spheres connected by tubes big enough to use as passageways. Exhibits were displayed in the spheres during the Expo, and visitors climbed stairs in the tubes from one to another. Today, only five of the spheres are open to the public, and they make up a museum of the 1958 fair.
Atlas Obscura tells the story of the Atomium, plus an earlier building from the Brussels International Fair in 1897 that was commissioned and built, but never open to the public until 2002! The building designer and the artist who decorated inside with a mural could never see eye-to-eye. See the mural and find out what the fuss was all about at Atlas Obscura. There are pictures of plenty more buildings built for various World Fairs in Brussels.
(Image credit: Flickr user Charles Roberts)