The network of railway lines and road tunnels beneath the elevated plaza of La Défense prevented the pillars supporting the arch from being exactly in line with the axis: it is slightly out of line, bending the axis should it be extended further to the west. From the roof of the Grande Arche, a second axis can be seen: the Tour Montparnasse stands exactly behind the Eiffel Tower. The Seine-Arche project is extending the historical axis to the West through the city of Nanterre, but with a slight curve.