>>12890985
there are very many books about Rome's tunnels and underground.
fascinating stuff.
there are some Mithraeums in Rome, in tunnels, those are known.
It's a hobby of mine: arm chair archiology.
the stuff you discover.
Look for videos of rome. One guy he's a spanish guy, so it's all in Spanish, he'll lower his camera down into a tnnel. He has a great video at Castle San Angelo, and others at the Forum and the Vatican.
He shows you some of what you ask about but in other parts.
I recently learned that through the Roman Forum there is a channel that was cut through to drain the area due to constant flooding. It was built with Tufa blocks 3 meters on a side! (quite a deal).
There is very much in Rome in full view that doesn't get a lot of write up in History or travel books.
use walk through videos. Learn how to translate Latin or Italian. Read the gravestones, monuments, and signs.
I was able to discover about a well known (in Rome) Roman family that way by reading a name that doesn't show up that much in the books I have.
but wow it's got a long and rich history!
so as far as what is underneath the Vatican?
it was built on a circus, was it not?
you can study it's archiology and even see 'time lapse' now and then kinds of videos.
or get a book with an overlay that will show the old circus, then the St Peters (as it was at first)
and then Michalangelos and Bernini's work too.
If you really want to know there are many books.
"Subterranian Rome" is one title