The term 'Paris Underground' refers to the Paris METRO, which is a rapid transit system in the French capital. It is the second busiest METRO system in Europe, with over 300 stations and 16 lines. The Paris METRO is known for its art nouveau entrances and its iconic Art Deco signage. It is an important part of the city's transportation network, providing millions of commuters with a reliable and efficient way to get around the city.