Mind Your Step Illusion By Erik Johansson, Sergel's Square, Stockholm, Sweden

• Final proposal
The hexagonal building ("Muttern"), a coffeehouse, was removed in 2005 in favor of a street level entrance to the metro station.
In 1957, a first official proposal presented a square virtually similar to the present; except that instead of the fountain there was an opening with tall trees and on the western side, where the flight of stairs is today, was a building was standing on pillars. The Chamber of Commerce was critical of the concept, concluding pedestrians on a lower level would produce poor business sites, an analysis which would eventually prove correct. Their own proposal the following year, developed together with various authorities, reserved street-level to pedestrians while cars were confined below ground. This counter-proposal was however produced in only two months, which made it easy for opponents to pin-down its weaknesses (mostly a failure to leave enough space for the metro which was being constructed at this time). Nevertheless, Helldén's proposal failed to impress the city as well, and Helldén together with other hand-picked experts was therefore sent on a tour around Europe, including Coventry and London, to find a better solution. In Stuttgart they could conclude that having pedestrians on a lower level required escalators, and in Vienna the pedestrians hall Opernpassage gave them the inspiration to replace the central open space at Sveaplatsen with a round restaurant with glass walls, an aesthetic device intended to give the square an architectonic dignity.
This newly introduced centre-piece resulted in a proposal for a fountain with a monument above it. For the shape of this fountain, Helldén consulted his friend, the mathematician and artist Piet Hein, who in less than in minute found a curve with a "continuously varying bending" and immediately named it the superellipse. Before presenting his final proposal in 1960, Helldén added the triangular pattern to the pedestrian plaza and the wide stairs on its western side.