Most people have no idea… Lockheed Martin in Georgia has tunnels all over city/state.
When I say tunnels, I'm talking six lanes wide, big enough for semi's carrying over-sized loads. Caught us completely off guard how vast it was underground vs above.
Some submarines are dry docked for maintenance under Crane Naval base, how the absolute fuck do you think they get them there? I was a child on a field trip, they never thought we'd ask that simple question, it's the only reason they allowed us to see it.
Now think about how many tunnel boring machines are sold per year, how fast they can produce a completed tunnel these days (damn things are quick) and who the hell is using them.
I think their is more going on underground than we can possibly conceive.