The “PATH” is truly an amazing feature of Toronto. I never really appreciated this urban marvel until I had to do a project for Marketing Geography (great course BTW), in which we had to analyze the stores from a section of PATH. The PATH stretches for 27 km, and has over a 1000 shops. If you’re new to Toronto, I highly suggest you take a stroll during the afternoon, for more details check out http://www.toronto.ca/path/
When I got my job downtown, I was kind of surprised to learn there was a vast underground network of walkable tunnels underneath the city. It sounds so outlandish. It is a good thing that they keep the molemen population in check, though, because that would put a damper on tourism.