RÉSO, commonly referred to as the Underground City (), is the name applied to a series of interconnected office towers, hotels, shopping centres, residential and commercial complexes, convention halls, universities and performing arts venues that form the heart of Montreal's central business district, colloquially referred to as Downtown Montreal. The name refers to the underground connections between the buildings that compose the network, in addition to the network's complete integration with the city's entirely underground rapid transit system, the Montreal Metro. Moreover, the first iteration of the Underground City was developed out of the open pit at the southern entrance to the Mount Royal Tunnel, where Place Ville Marie and Central Station stand today. Though most of the connecting tunnels pass underground, many of the key passageways and principal access points are located at ground level, and there is also one skybridge (between Lucien-L'Allier Metro station and Gare Lucien L'Allier). In this regard, the Underground City is more of an indoor city (ville intérieure) than a truly subterranean city, although there are vast commercial sectors located entirely underground. The network is particularly useful during Montreal's long winters, during which time well over half a million people are estimated to use it every day. The network is largely climate controlled and well-lit, and is arranged in a U-shape with two principal north–south axes connected by an east–west axis. Combined, there are of tunnels over of the most densely populated part of Montreal. In total, there are more than 120 exterior access points to the network, not including the sixty or so Metro station entrances located outside the official limits of the RÉSO, some of which have their own smaller tunnel networks. Some of the city's larger institutions, namely McGill University, the Montreal Museum of Fine Arts, Concordia University and the Université de Montréal, also have campus tunnel networks separate from the Underground City. Overview In 2004, the downtown network of the underground city was re-branded and given the name RÉSO, which is a homophone of the French word réseau, or network. The "O" at the end of the word is the logo of the Montreal Metro. Schematic maps bearing the RÉSO logo are found throughout the network. The largest and best-known segment is located in the centre of downtown, delimited by the Peel and Place-des-Arts Metro stations on the Green Line and the Lucien-L'Allier and Place-d'Armes stations on the Orange Line.