RÉSO, commonly referred to as the Underground City (French: La ville souterraine), 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.

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).

The underground city is promoted as an important tourist attraction by most Montreal travel guidebooks, and as an urban planning achievement it is impressive.

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.

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.

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).

[citation needed] Also, the construction of a tunnel between Eaton Centre and Place Ville-Marie consolidated the two central halves of the underground city.

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.

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 1.7 km (1.1 mi) as the crow flies, or approximately 3 kilometres (2 mi) 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.

[3] 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.

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