The PATH is an underground mall that links many of the office towers in downtown Toronto together. It extends from Union station at Front street at least as far North as Dundas street, possibly further. In some places it is a full blown mall filled with shops and other mall type stuff, in others nothing more then a poorly lit hallway. The real target audience of most of the PATH is office workers and through the work week it is packed to the gills with them.
Getting into the PATH is easy enough, there are lots of entrances above ground and in the lobbies of buildings that are connected, all labeled with the distinctive PATH logo. Once inside the thing to remember about PATH is that it is underground, so all the things like the Sun and street signs that would normally help you navigate are gone. This makes it exceptionally easy to get lost in the PATH. If you have to go somewhere in a hurry, bite the bullet and go outside, it will be faster. If you are new to the whole ordeal, having a more experienced person guide you is a hell of a good idea. Oh and perhaps a trail of bread crumbs.
In spite of it's cave like qualities, the PATH walkway provides a wide range of shops and services. It also links together several food courts to provide a vast range of very similar fast food to the army of office workers toiling away above it. 10Km (while I am not sure that something that isn't linear has a length per say) is a lot of mall. Odds are that if you forgot to bring something to work; lunch, deodorant, pants, you can find it in the PATH, all it takes is money.
Provided you know where you are going in this underground warren, it is a great way to stay out of nature's way and avoid all the sun, rain and slush that downtown Toronto can dish out. Connected to the TTC in several places as well as Union Station(GO transit, Via Rail) and the Greyhound bus terminal you can get into, out of and about downtown Toronto without going outside at all. Why you would want to do this if you weren't an office worker is beyond me, but the option is open to everyone, suit or no suit.
If you want to do something really creepy though, go into the PATH on the weekend. It is very strange to see a mall that is clearly designed for several thousand people, deserted at noon an a Saturday when every other mall on the planet is jammed solid with people.
A map of the PATH Walkway is available at: http://www.city.toronto.on.ca/path/