The Underground City
Montreal's Underground City refers to the maze of malls and concourses that connect major Downtown buildings, but it's a bit of a misnomer.
It's not really a city (nobody lives here) and a good deal of it is not underground either.
Perhaps a better metaphor would be to think of it as an enclosed archipelago: the malls are the islands and they're connected causeway-like passages as well as ferries (the metro).
Either way the system is huge and stretches some 32 km (20 miles) to cover 4 million square meters!
The result is that there's often a lot more bustle underground than at street level; particularly in winter.