Montréal’s underground system isn’t particularly extensive, but it does have a twist: it’s attached to a system of pedestrian friendly tunnels and underground spaces.
Sometimes called the RISO (I’ve never heard anyone call it that) or the Underground City, it’s not so much a city as a series of interconnected shopping spaces, food courts, entertainment areas, etc. Imagine if ants were concerned with shopping, and you have an idea of the underground city.
I’ve not spent a lot of time in it yet — it wasn’t that cold a winter and I had no desire to get out of nice cold air and into, well, a mall, but I have been finding spaces in it, and it is really special.
And… it isn’t always underground, but it is always under a roof.