Probably don’t need to actually do all that much digging. From what I understand. Most major cities have quite a bit of forgotten infrastructure and subterranean shelters just beneath the surface. Just gotta dig far enough to connect them as opposed to an entirely new tunnel system.

Forgotten Sewer Lines. Basements that were never properly filled in when the property they were formerly connected to was bulldozed. Maintenance tunnels to access infrastructure that’s now considered ‘outdated’ and so while technically not ‘forgotten’ isn’t actually maintained or kept track of. Unfinished Subway/Train lines that were never opened to start with. Closed Subway or Train Lines that never reopened. Prohibition era Rum Runners tunnels in some costal cities. Abandoned Industrial Complex’s as well as its not unheard of for some particularly large facilities to have a portion below ground level. Some places also have abandoned Civil Defense and Fallout Shelters from the Cold War as well.