The construction of the Atomium was a technical feat. Of the nine spheres, six are accessible to the public, each with two main floors and a lower floor reserved for service. Tubes of diameter connect the spheres along the 12 edges of the cube and all eight vertices to the centre. The central tube contains the fastest elevator of the time with a speed of , installed by the Belgian branch of the Swiss firm Schlieren (subsequently taken over by Schindler). It allows 22 people to reach the summit in 23 seconds. The escalators installed in the oblique tubes are among the longest in Europe. The biggest is long.