Well, I am no expert for sure but the area around the Pantheon appears to be at or near its ancient level and that is despite the flooding that has occurred. Flooding that he contends helped to bury the rest of the city ruins.
The back side of the Pantheon has an arch or butress from an old bath and the right side of the Pantheon[viewed from the front] has a pretty clear view of the foundation itself.
There is also a temple to Hadrian, I think it is, on one of the walks to the Trevi fountain which is nearly at street level.
I guess I am wondering if the over burden of the more modern buildings is not the major cause of the depth of burial of the old sites. It is row after row of connected buildings constructed from the fall of Rome to modern day on street after street in the central storico. Seems to me that dirt had to go somewhere.
As I said, it’s been in constant use. Other parts of Rome got buried. One of the Renaissance-era artists tried to excavate a basement under his house and rather shortly hit a large flat stone surface. He excavated out to the edges, then around the stone. It turned out to be the capital of a still-standing Roman-era column, and much of the rest of the structure was intact.