system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why were some ancient cities needed to be excavated? How does a city get lost due to "something" covering it?

For instance, consider the city of Pompeii, which was buried under the ash from a volcanic eruption. What does it exactly mean? Did the entire city get covered and formed a plateau-like structure? How could an eruption cover a big area and not cover other surrounding places?

Also, I was reading about cities in the Indus valley civilization and even in the Babylonian civilization, they too needed some excavation after which the cities were revealed.

Are a lot of current cities over some ancient city?

I'm sorry for a lot of questions, but I'm just curious with a lot of questions on that topic.
gpt
In many cases it's buildup of leaves,plants and dust that settles over decades-centuries and slowly covers everything. There is also a sinking action the soil can play when conditions are right and some buildings may sink from the bottom and be covered with plants and dust from top.

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Eventually a hill forms and people forget there was a city at all.