Formation of underground layers of Rome

Question

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.

Answer 1

Did you ever hear of a tell? In archaeology a tell is an artificial hill formed by generation after generation, century after century, of people living in the same place.

A tell is an artificial hill created by many generations of people living and rebuilding on the same spot. Over time, the level rises, forming a mound.[9] The single biggest contributor to the mass of a tell are mud bricks, which disintegrate rapidly. Excavating a tell can reveal buried structures such as government or military buildings, religious shrines and homes, located at different depths depending on their date of use. They often overlap horizontally, vertically, or both. Archaeologists excavate tell sites to interpret architecture, purpose, and date of occupation.

A classic tell looks like a low, truncated cone with sloping sides[3] and can be up to 30 metres high.[4]

https://en.wikipedia.org/wiki/Tell_(archaeology)1

In Rome, the Baths of Trajan, opened in AD 109, were built over the first floor of an important wing or range of the Golden House of Nero built about AD 64 to 68.

Some of the really old churches in Rome that are 1,500 year old are surrounded by ground much higher than their floors so you have to walk down steps to the front doors. At other churches one or two older churches are buried beneath the present ones, and in some cases tourists can tour those basement churches which were at ground level when they were built long ago.