3. Atomium, Brussels, Belgium
Originally built for the 1958 Brussels World Fair, the Atomium was designed by André Waterkeyn, an engineer, and André and Jean Polak, architects. The stainless steel building is 335 feet tall (102 m) and has nine spheres connected together in the shape of a unit cell of an iron crystal.
Tubes connect the spheres along the dozen edges of the cube, as well as all eight vertices to the center. Escalators and elevators in the tubes connect five usable tubes, which are often occupied for public events. The four highest spheres are not safe and are therefore off limits to people.