I'm planning a trip to Rome and one of the interesting things about it are how the city is just layer upon layer of history. For instance, you can go to the [Basilica of San Clemente] (https://en.wikipedia.org/wiki/Basilica_of_San_Clemente_al_Laterano) and see a 12th century church on top of a 1st to 4th century basilica that's on top of a Roman Republican-era Patrician home - meaning that Ancient Rome street level is significantly lower than it is today. How does this happen? Did they not know at the time that there was a building under there or did they just think "Hey, there's a church. Let's build another one on top of it"?

And a related question - how exactly does the ground level rise like that? The Pantheon (1st century) is just a few blocks away from [Largo di Torre Argentina] (https://en.wikipedia.org/wiki/Largo_di_Torre_Argentina) (Republican period), but the Largo di Torre Argentina ruins are about 20 feet below street level. What accumulated in that area to bury those ruins? They must have had some level of control over it since the Pantheon is still at ground level a few blocks over due to it being in near constant use.