Underground City

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.

Source: Wikipedia