Keep an eye open for the beautiful Métro entrances designed by Hector Guimard at the beginning of the 20th century. For a time the Art Nouveau style of these stations was viewed as passé, and many were destroyed in the name of ?modernization.? Fortunately, more than 50 survive (often in remote and non-touristy areas).
In 1898, as the Métropolitain was being constructed, a competition was launched for the design of station entrances. Guimard won the competition and construction took place through the turn of the century. When Guimard proposed his design for the Métro entrance in front of the Opéra, it was criticized for not blending with the style of the building. In a huff, Guimard quit his contract and designed no additional stations. This explains the rather unusual classical style of the Opéra Métro station.
Pictured here is the station Palais Royal, next to the Louvre.
For metro fans, a few interesting web pages:
http://www.metro-paris.net/metro.html
(an amazing page that includes the story behind the name of each Metro station, the history of the Metro, maps, and a link to a route planning guide)
http://www.greatbuildings.com/buildings/Paris_Metro_Entrances.html
(more on Guimard)The vegetal beauty of these stations has been recognized around the world. The National Gallery in Washington DC displays an original Guimard Métro entrance. The Montréal Canada Métro uses an original Guimard station, donated by the Paris RATP (the transportation organization that runs the Métro).

