Rose Ligne
In the Da Vinci Code Dan Brown mentions a line, that crosses Paris from north to south: 'La Ligne Rose'.

It is true there are 135 rond plates as pictured on the photo above. The plates represent the 'zero meridian' from Paris from north to south. La Ligne Rose is not so old as you might expect. The plates were made by the Dutch artist Jan Dibbets in 1986 in memory of the French fysician Francois Arago.