The RESO — Montreal's underground city — is 33 kilometres of interconnected tunnels linking the downtown metro stations, hotels, shopping centres, office towers, universities, and cultural venues. It was not designed as a network; it grew piece by piece from 1962 onward as individual buildings connected to each other and to the growing metro system. The result is a largely unplanned labyrinth that on cold winter days carries more pedestrians than the streets above.
As a practical matter, the underground city means you can walk from Berri-UQAM station to Bonaventure station — passing through Place Ville-Marie, the Bell Centre, and dozens of other landmarks — without once setting foot outside. In January, when temperatures drop to -20°C and the wind chill is brutal, this is not a small thing. The underground is also a shopping destination in its own right: the Promenades Cathédrale beneath Christ Church Cathedral, Montreal Eaton Centre, and Place Montreal Trust are the main retail clusters. Getting genuinely lost here is easy; going wherever your instincts lead is a reasonable strategy.