Chicago has tunnels everywhere…
Many connect to the Deep tunnel project
we played in them when I was a kid w/bb guns and flashlights. they run from downtown to past O'hare airport, they are huge you could fit a 747 inside the big ones.
They flooded many of them on accident when they were setting some pylons in the in the Chicago river and breached some old tunnels.
It caused a huge power outage for days in the city.