Few cities in the world are more identified with their subway systems than Paris. One of the busiest metro systems in the world, it carries more than 4 million riders a day on some 16 lines to 300 stations. The Chatelet stop in Place St. Opportune, in the heart of Paris was one of the very first stops built for the very first line in 1900. It’s also the world’s largest metro station, with five metro lines and three commuter lines running through it. One of the original archways keeps the art nouveau metropolitan style that’s the hallmark of the Paris Metro.

Standing inside the station, author Mark Ovenden says the style was the creation of “an amazingly young architect” named Hector Guimard. “He had the idea of building things out of this wonderfully laced wrought iron that looks like it’s kind of grown there, almost organically — the way that trees grow or that plants grow. It’s a very organic-looking form and quite advanced for its time.”

Ovenden is co-author of the book Paris Underground: The Maps, Stations and Design of the Metro. He believes the contrast between the Metro’s ornate entrances and the formality of the surrounding boulevard gave the city some sense of nature. That might be one reason that Parisians grew fond of their Metro so quickly, he says. “They were also so totally unique.”

In the late 1800s, Paris was well behind London and New York in building a metro system. Visitors to the 1889 Grand Exhibition that featured the Eiffel Tower had to ride a horse and buggy to get there — an embarrassment that spurred the completion of the Line 1 in time for the World’s Fair in 1900. Built in 20 months, this first line of the Metro connected all the major Paris attractions.