I have recently came across several news articles (for example) from various decades, talking about existing underground structures in the city of Rome, which used to be above ground at some point of settlement's history.
I understand that the layering happens due to a number of factors, like Tiber flooding the area, hills of Rome eroding over time, and human-made by-products of civilization piling up.
However, a couple of things aren't quite clear for me.
1) Are some of these ruined buildings actually structurally intact? Specifically, I am confused by certain allegations some articles have made, creating an impression that the older city is literally just sitting there, under a layer of soil, filled with dirt.
2) Assuming the building doesn't just disappear into a sinkhole overnight---how and why would ancient citizens build over them? Are there any historical accounts of this process/implications of having certain buildings be covered so much, you can't even get into the door?
3) Is there any art of a crossection of any such case of intact building under a newer one?
Specifically, I am not talking about basements and cellars, I am talking about surface level structures.