I was amazed by the extent of this place. At first I thought it was just an underground shopping mall, but quickly realized that it's function was to cut out passage through surface streets altogether in the winter time. The large atriums let plenty of light in, even if you're subterranean. There are numerous food courts and plenty of good shopping.
The passageways connect you to hotels, conference centers, subway stops, cultural centers.
It's supposed to be 2 miles of passageways but feels longer. We walked all the way to one end, at the Westin to reach old Montreal without stepping outside. Amazing.