The catacombs in Paris have sidewalks and ruins of buildings and are now completely covered by the modern city. Also under Manchester… I don’t get how a city used to exist and then another one is simply built on top. Let me know, thanks!
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The catacombs were cut as tunnels, mostly to harvest limestone.
Other than that. In the old days, it took a great deal of labor to haul stuff away. When old buildings were torn down (or fell down) and were repurposed, any usable material was scavenged for reuse, and the rest of the building would be mashed flat, filled in, and the new building built on top.
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