Guimard's Paris underground station entrances are well known. The architect designed them in 1899 and they are a clear example of the total art as it was seen by Art Nouveau authors.
The whole town seen as the ground upon which one could build the New World, where beauty and function could walk side by side.

The iron work defines wavy lines, that remind us of flower stems, even something organic, fleshy, to my friend Paolo they remind him of Chtulhu!.

I like them so much. I think this is a wonderful way to enter the city belly.
Another famous underground station is the one at Porte Dauphine.

Among other stations built by Guimard (many were destroyed and replaced) I remember: Louvre-Rivoli (Paris I)   Louvre-Palais Royal (Paris I)   Chardon-Lagache (Paris XVI) well kept and scarcely known.

Many Underground entrances are reviewed in the page dedicated to the Paris Metro entrances.