>>40869816
Another thing to keep in mind, is that these people probably had technology that is difficult for us to conceptualize at all, the last few thousand years of our civilization has been very dependent on metal and fire, with the industrial revolution and early 20th century representing the zenith of this technology. Metal seems to be entirely absent from these ancient remnants, now maybe that is because it was all scavenged and recycled, but if the tech was similar to ours there would be artifacts too large to break down by simpler successor cultures and alloys they could not melt with primitive means.

The number of dark underground sites also seems to imply that they had a way of lighting these spaces that wasn't based on using fire to produce light. A lot of people have suggested some kind of electromagnetic based technology, which seems more plausible and also might explain the weird geometries and specific choices of stone. We also have only had electrical technology for around 200 years, so there might well be a lot to still discover, and the use of semiconductors is even more recent, since our culture started out with metal mechanical tools and the first sources of electricity were chemical batteries (based on reactions of metals and acids), so electrical technologies were first applied to metal conductors. There might be a whole field of litho-electrical tech that we haven't even begun to wrap our heads around.