When Place Ville-Marie, the cruciform skyscraper designed by I. M. Pei, opened in the heart of downtown in 1962, the tallest structure of the time also signaled the beginning of Montréal's subterranean city. Montrealers were skeptical that anyone would want to shop or even walk around in the new "down" town, but more than four decades later they can't live without it.
About half a million people use the 30-km (19-mile) Underground City, or la ville souterraine, daily. The tunnels link 10 métro stations, seven hotels, 200 restaurants, 1,700 boutiques, and 60 office buildings—not to mention movie theaters, concert halls, convention complexes, the Centre Bell, two universities, and a college. Those who live in one of more than 2,000 connected apartments can buy milk on a February day and never have to put on their coat.
Most of the Underground City parallels the métro lines. The six-block sector of continuous shopping between La Baie (east of the McGill station) and Les Cours Montréal (west of the Peel station) is perhaps the densest portion of the network. The first link in the system is at Place Ville-Marie, which is part of the main hub and a good place to start exploring. From here you could cover most of the main sites—from the Centre Bell to Place des Arts—without ever coming up and without having to take the métro.
There aren't any bad times to visit—of course it's busy at rush hour and during the holiday season—but there are certainly good times, such as during a snowstorm or in the heat of summer. While Montréal is generally a safe city, note that some of the farther reaches of the network can become desolate at night.