As far as I know, archaeological excavation discovered several layers of the city of Troy, each one built on top of the ruins of the older ones.
I just can imagine how that might have happened.
One possibility: The old ruins are so completely covered by mud or sand that the people building the new city just do not notice them. Or if they notice, they do not care since the old stones deep below ground do not inconvenience them, even when digging cellars.
That might happen once or twice I guess, but several times seems very unlikely to me. How long does it take for a layer of mud or sand to reach the necessary height?
Another possibility: The old city is destroyed or abandoned, but the ruins are still visible above ground. What would new settlers do? Either remove the old stones to get level ground for new buildings, or even use the old stones for the new buildings. Either way, hardly anything would be left of the old city, and very little for archeologists to dig up.
So that seems also unlikely.
What am I missing, how did it actually happen?
Thanks for the link to Formation of underground layers of Rome, that's almost a duplicate I guess, but since my basic problem is not answered there, I'll try to state it more precisely:
The answers so far seem to suggest the following process: The ruins are still visible, but the ground around them has risen several feet. Digging up the old stones would only create a deep hole in the ground which had to be filled again if you want a building on the new ground level: That is considered too much effort, especially if the old stones are considered not very useful as building material anymore (For mud-bricks I get that, but for Roman stones it is harder to understand). So the rubble is levelled into a flat building site and the new building is erected on top of it.
That leaves the following points unclear to me:
1) If the rubble is levelled, can it really form a stable building site? I can only imagine that if the old stones are covered with a layer of earth to form a flat surface. Doing that seems unlikely if the new surface is much higher than the ground around the building.
2) Would levelling the old rubble not destroy most archaeological traces? Why is still so much left to discover?
3) If the earth around the building rises higher and higher over centuries: Why exactly does that happen? Does it happen also outside cities? If only in cities: Is the accumulating dirt brought by wind or river floods? That seems unlikely for cities built on hills. Or is it formed by human waste? How can I imagine that: Garbage heaps beside the streets getting higher and higher and composting into earth that spills over onto the street itself where it is simply trampled down so that the street level rises, and the inhabitants don't care? What if the street is paved?