Excuse me if I haven't formulate the words in the title or the text correctly.

I was wondering, how some old buildings and artefacts ended up under the surface of the Earth? Is the Earth somehow slowly swallowing them? If so, I would understand for some very old artefects. But some Roman buildings that are found, are near or directly IN a city.

I got this question in mind, because of the dionysus mosaic in Cologne, Germany. It is in the center of the city, and even before it was put there it was made, Cologne was always a city. The house it was in, was enormous even for today's standards. So how can the mosaic just disappear underground, only to be found during WW2 diggings?