Okinawa is the largest island in the Japanese Ryukyu archipelago. It contains strange structures that at first glance resemble something modern, but are called tombs.
It is known that such tombs were used in the Middle Ages, although there is no information about burials either, but this is not so interesting.
If you look at the entrance of the "tomb", it would seem that there is nothing special, you can see that it is ancient, and no processing is even close to be seen, but you cannot say that about the upper part.
The upper part, if it can be called a roof, is made figuratively, with smooth lines, transitions. In other words, the entrance to the "tomb" and the upper part are something completely different, as if created by different civilizations.
I have looked at about 10 sources, but as a rule, each one copies from the other. There is no information about the exact age of the "tombs", in fact, as well as pictures, not very much.
But even a few pictures are enough to draw certain conclusions. Proceeding from the fact that not a few ancient civilizations built something on the ruins of a possible highly developed civilization, then it is most likely the same here.
Promotional video:
The upper part is not only perfectly finished to a smooth state, but also has a convex shape with smooth transitions to other parts. In general, I'm almost sure that the upper part, in particular, was not built in the Middle Ages, but much earlier. And only then, people adapted some of the remains of the building for tombs and, possibly, they completed something.
There is very little information to judge anything, but a comparison of the two parts of the structure makes it clear that this was done at different times and at different levels of civilization development.