Architect Hector Guimard was the principal designer of the Paris Métro system, which opened in 1900 at the time of the Exposition Universelle. His work is associated with art nouveau, a style of art and architecture that is based largely on organic forms from nature. Guimard's designs were meant to clearly mark the new subway entrances and make the novel form of mass transportation more attractive to riders. The three entrance styles he designed were industrially produced in cast iron until 1913. The entrances became so iconic that Parisian art nouveau came to be known as le style Métro and le style Guimard. This version, with its graceful upward reaching tendrils and vines, can still be seen at 86 station entrances in Paris today. Between the 1930s and 1960s, the Métro removed a number of Guimard entrances in poor condition and sold some to collectors and museums who restored and displayed them. In 1978 the remaining intact Guimard entrances were registered in Paris as Monuments Historiques.