Монреальдің Жер Асты Қаласы (Montréaldiń Jer Asty Qalası)

Network of interconnected buildings in the central business district of Montreal, Quebec, Canada

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, Université de Montréal and the Université du Québec à Montréal also have campus tunnel networks separate from the underground city.

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. The underground city is promoted as an important tourist attraction by most Montreal travel guidebooks, and as an urban planning achievement it is impressive. For most Montrealers, however, it tends to be considered more as a large mall complex linking Metro stations—they may not even know they are in it. Many Canadian cities have some kind of tunnel or skywalk system downtown to help people avoid the weather. Most parts of the Montreal underground city are open while the Metro is in operation (5:30 AM to 1:00 AM), though some are closed outside of business hours. Maps of the underground city and the Metro can be obtained free of charge from all Metro stations, and the network of buildings is indicated on most maps of the downtown core. Nearly 500,000 people use it per day. It is also the largest underground complex in the world. It stretches for 32 kilometers (20 miles) and covers 4 million square meters (one and a half square miles). According to official statistics, its corridors link up with 10 metro stations, 2 bus terminals, 1,200 offices, about 2,000 stores including 2 major department stores, approximately 1,600 housing units, 200 restaurants, 40 banks, movie theatres providing 40 screens and other entertainment venues, 7 major hotels, 4 universities, Place des Arts, a cathedral, the Bell Centre (home of the Montreal Canadiens), and 3 exhibition halls: the Place Bonaventure, the Convention Centre (Palais des Congrès de Montréal) and the Olympic Centre.

The vision for the underground city was originally that of urbanist Vincent Ponte, for whom a commemorative plaque was unveiled in November 2006 at Place Ville Marie. The first link of the underground city arose with the construction of the Place Ville Marie office tower and underground shopping mall, opened in 1962 and built to cover an unsightly pit of railway tracks north of the Central Station. Two tunnels linked it to Central Station and the Queen Elizabeth Hotel. The advent of the Montreal Metro in 1966, in time for Expo 67, brought tunnels joining Bonaventure station to the Château Champlain hotel, the Place du Canada office tower, Place Bonaventure, Central Station, and Windsor Station, forming the core of the Underground City. Square Victoria OACI station connected to the Tour de la Bourse, Montreal's stock exchange building. Adding to the development of the underground city was the Montreal Urban Community Transit Commission's policy of offering the aerial rights above Metro station entrances for construction through emphyteutic leases, an advantageous way to acquire prime real estate. When the Metro began running in 1966, ten buildings were already connected directly to Metro stations; development would continue until only three free standing station entrances (Square Victoria OACI, St Laurent and Place des Arts) remained in the central segment. In 1974, the Complexe Desjardins office tower complex was constructed, spurring the construction of a "second downtown" underground city segment between Place des Arts and Place d'Armes station, via Place des Arts, Complexe Desjardins, the Complexe Guy Favreau federal government building, and the Palais des Congrès (convention centre). Between 1984 and 1992, the underground city expanded, with the construction of three major linked shopping centres in the Peel and McGill Metro station areas: Cours Mont Royal, Place Montréal Trust, and the Promenades Cathédrale (built underneath Christ Church Cathedral). McGill station was already linked with The Bay, Eaton's (now the Complexe Les Ailes), Centre Eaton, and two other office/mall complexes. Between 1984 and 1989, the underground city grew from of passages to almost
Mega projects added to the size of the network throughout the 1990s, including Le 1000 De La Gauchetière (the tallest building in Montreal), Le 1250 René Lévesque, and the Montreal World Trade Centre. Although these buildings have only a secondary commercial sector, they use their connection to the underground city as a selling point for their office space. Also, the construction of a tunnel between Eaton Centre and Place Ville Marie consolidated the two central halves of the underground city. The construction of the Bell (originally Molson) Centre connected Lucien L'Allier Metro station to the underground city, as well as replacing Windsor Station with the new Gare Lucien L'Allier commuter train station. Most recently, in 2003, the complete redevelopment of the Quartier international de Montréal consolidated several segments of the central underground city with continuous pedestrian corridors. The construction of the ICAO headquarters joined Place Bonaventure to Square Victoria OACI station, which in turn was joined to the Palais des Congrès and Place d'Armes station via the new Caisse de dépôt et de placement building and a tunnel under Place Jean Paul Riopelle. Uniquely, the new tunnel sections in the Quartier International contain educational and artistic displays sponsored by major Montreal museums. As a result of this construction, one can now walk all the way across the centre of downtown, from the UQAM Sherbrooke Pavilion at the corner of Sainte Famille Street and Sherbrooke Street to the Lucien L'Allier Metro station just south west of the Bell Centre, without going outside—a span of as the crow flies, or approximately walking distance.

On Friday, August 24, 2007, construction crews discovered a seven meter (23 foot) long fissure in the ceiling of an underground corridor linking the McGill station to The Bay store located under de Maisonneuve Boulevard, between Aylmer Street and Union Avenue. The station, the Underground City shops, and above ground streets and buildings were closed to assess whether there was any risk of collapse of the structure. Service on the Métro Green Line was halted between Berri UQAM and Lionel Groulx stations until Sunday evening. According to a spokesperson for the Hudson's Bay Company, city workers may have caused the damage by hitting a nearby pillar. During the weekend, city workcrews worked non stop to shore up the sagging slab of concrete, installing more than 1,000 temporary metal supports. On Monday, August 27, 2007, service was restored to the Green Line, and all streets but the block of de Maisonneuve boulevard between Union and Aylmer were reopened to traffic. The one block that was not open to traffic was open to pedestrians. All buildings reopened, including The Bay. Officials said that it would take months to fix the problem. While inspecting the site, it was discovered that 2021 Union, the Parkade Montreal building, was in danger of having concrete side panels fall off. City engineers performed emergency repairs. A report later blamed the construction of a bike path for the damage. Street traffic on De Maisonneuve resumed in March 2008.

The central segment interconnects the following seven Metro stations via indoor walkway. As the Berri UQAM station, which allows transfers between the Green, Orange and Yellow lines, is two Metro stops from the closest station in this segment, in many cases it is quicker to walk than to take the Metro. The lists of connected facilities which follow are grouped by segment and nearest Metro station.

Every February, Art Souterrain presents a journey through the central segment of the Underground City during the Montreal Highlights Festival, showing contemporary artistic projects.