Explain this "building over a city" thing to me.

Factual Questions
1

It’s becoming increasingly common to buy a house, raze it, and build something bigger and better. Or blow up a sports stadium to build a mall, or implode one casino to build another. In each of these cases one building is destroyed, the grounds excavated, and something new built up and, quite often, the new foundations are more extensive than the old.

This makes sense to me.

While wiki-ing around I came across a description of a Viking meadhall:

Every now and again I’ll read a reference about a modern city that was “built over” an ancient city.

This doesn’t make sense to me, probably because of my modern sensibilities.

1000 or 3000 or 5000 years ago, when they tore something down - didn’t they excavate the site? The idea of “building over a city”, to me, is that they just kept building up while the original foundations kept sinking, and if we continue to do this all cities will eventually be the size of Mount Everest.

Of course, I live in the upper Midwest where 4’-8’ foundations are de de rigueur to prevent frost heave and everyone has a tornado-resistant basement, and perhaps I’ve just answered my own question, but I’ve gotten this far so I’m going to finish:

What do archaeologists mean, exactly, when they say that City Y is “built over” City X?

2

The Master speaks.

3

Also, don’t earthworms constantly burrow through the soil and then deposit castings on the surface, eventually leading to everything getting buried with the passing of years?

4

You should watch an episode of Time Team. They encounter that kind of thing a lot, and it’s often because the older building will be expanded on, and in doing so some of the smaller connecting rooms will be knocked down, and newer ones added on.

But there will also be cases of essentially the same intent, but the whole building is knocked down first (or was burned down for whatever reason). The foundations are buried under a layer of soil, and new foundations are laid on top.

5

Fighting squalor since ancient times
(Its taking longer then we thought)