There is a large underground city. Europeans think the whole island is the underground city but, in fact, it spans about 6km x 4km.. which is still pretty big and you can walk for hours. More details here :

map from : https://montrealsouterrain.ca/ 

For parking, be sure to download the mobile app to pay parking meters from your phone! http://www.statdemtl.qc.ca/en/practical-info/pmobileservice.html