For a modern example, see the Raising of Chicago: https://en.wikipedia.org/wiki/Raising_of_Chicago. I think I've seen an image of what the old ground level looks like now, and / or that you can take tours there. There's probably loads of stuff buried there now.
My understanding of the Chicago case is that most of what the old "ground level" was is just sewers today. I heard some of it was railroad tracks and maybe some are still used for cargo-only trains, but most of Chicago wasn't really raised a full "floor" with enough headroom for interesting underground spaces.
Some of Cincinnati's underground exists from plans to build subway trains that never completed. I think that makes Cincinnati's particularly sad being that it constitutes a perpetually unfinished public works/public transportation project.
Relatedly to that, Atlanta also has a tiny underground leftover from passenger train lines that ended passenger travel decades ago (and so was turned into a mall, because America): https://en.wikipedia.org/wiki/Underground_Atlanta
A US city often overlooked for some intricate people explorable underground spaces is Cincinnati: https://www.visitcincy.com/blog/post/unmistakably-cincinnati...
Some of Cincinnati's underground exists from plans to build subway trains that never completed. I think that makes Cincinnati's particularly sad being that it constitutes a perpetually unfinished public works/public transportation project.
Relatedly to that, Atlanta also has a tiny underground leftover from passenger train lines that ended passenger travel decades ago (and so was turned into a mall, because America): https://en.wikipedia.org/wiki/Underground_Atlanta