Atlantis? Fallen civ? Aliens?
One of Scaroth’s splintered selves.
The most popular theory is an earlier much more advanced civilization. Much much earlier than allowed to be suggested, and that these oldest works were already there when the later cultures came there. They just claimed them as their own and continued to build over them. But the later technologies just cannot rival the previous technologies and this is the hard part to explain, and because it cannot be explained it is just denied. But the same is found in South America also, with very striking differences of technology from the older to the newer.