Yes, the floor was protected through the centuries by meters of very soft clay and silt, supposedly from a catastrophic flooding by the nearby river Moselle, and the area doesn't exhibit tectonic shifts. They basically dug it out, cleaned it up a bit, and built a museum over it.
The floor is from the 3rd century A.D. and was discovered in 1852. It's in its original state; 7 of the 8 depicted scenes are complete, only the 8th scene was lost and replaced with a modern inscription (top left of my photo above the fountain).
Very few non-local people seem to know about this unique historic artefact; there is very little tourism. There is a Wikipedia page, unfortunately only in German, but perhaps you'd like to run it through a translator: