I’ve always admired the work of Roma Sotterranea. These guys search out old ruins that exist underground. They’ll take you on tours of them when possible.
Some of the sites are fantastic, but are really just a part of somebody’s basement. There’s where the trouble starts. If you know you have a Roman temple under the house that’s been in your family for 500 years that few people know about, you tend to keep it quiet. Believe me, you don’t want archaeologists digging under your abode. I’ve been one. You’ll wake up and all your beer will be gone. Then there might be a bill for all the work, compliments of some government agency or other.
But one by one these sites come to light. They’re fascinating to visit. One of my faves is below the 12th century Basilica of San Clemente I wrote about in Underground Rome
As we revise the article, the map has disappeared.
You may wish to visit Under Rome: Visiting the Ancient Houses Under Basilica Santi Giovanni e Paolo