"It's perhaps easier to explain the ones under the pyramids, as that location seems to be the original and the artwork goes back extensively."

In ways that raise questions about how long people like them have been around. The influence they might have had on cultures, assuming they weren't rules themselves. But also, where are the others now? What happened to those before?

"Harder to explain for the cities. It would speak to an impressive amount of fore-planning. Or deliberate moves to ensure they would be accessible."