Montreal’s Underground City (Montréal Souterrain) is the largest underground complex in the world. A labyrinthine maze of downtown tunnels connecting malls and hotels, offices, museums, banks, universities, and seven metro stations, the complex holds its own during the cold Canadian winter months, when over half a million use the space every day. There are 120 access points to Underground City, and with 80% of downtown Montreal’s office and commercial space connected via over 20 miles of tunnels spread over 4.6 square miles, as an urban planning achievement it’s impressive.
You can get maps of Underground City for free from all of the metro stations, and you might need one. This place is huge and practically a city in itself with some 5,000 stores, restaurants, boutiques, theaters, and connections to everything from a church to a hockey and ice skating museum.