From the Wikipedia: Built for the 1958 Brussels World's Fair (Expo '58), the 103-metre (335-foot) tall Atomium monument represents a unit cell of an iron crystal (body-centred cubic), magnified 165 billion times, with vertical body diagonal, with tubes along the 12 edges of the cube and from all 8 vertices to the centre.
Nine steel spheres 18 metres in diameter connect via tubes with escalators as long as 35 m, among the longest in Europe. Windows in the top sphere provide a panoramic view of Brussels. Other spheres have 1950s exhibitions. Three upper spheres lacking vertical support are not open to the public for safety reasons.