montreal has a shit load of "undergrounds", but they're all promoted like normal raves. The same venues host different undergrounds every week. They bring out headliners and shit to and they play at the "undergrounds".
I was at one that got run in by the cops. They didn't break up the party, just wanted to make sure the new smoking bylaw wasn't being violated.
We had about 100+ people on the bottom floor smoking and had to sit quiet as a mouse until the cops left. It took like 1hr. Fun stuff.
I'm pretty sure this here is a legal venue, but i've heard it's dangerous as fsck. I wanted to hit up a party at this joint so bad. It's an old bandoned bomb shelter. It's called "Mont Radar".