Scott Alexander in Astral Codex Ten:
Imagine a postapocalyptic world. Beside the ruined buildings of our own civilization – St. Peter’s Basilica, the Taj Mahal, those really great Art Deco skyscrapers – dwell savages in mud huts. The savages see the buildings every day, but they never compose legends about how they were built by the gods in a lost golden age. No, they say they themselves could totally build things just as good or better. They just choose to build mud huts instead, because they’re more stylish.
This is the setup for my all-time favorite conspiracy theory, Tartaria. Its true believers say we are those savages. We live in the shadow of the Taj Mahal, Art Deco skyscrapers, etc. But our buildings look like this: [See photo.]
So (continues the conspiracy) probably we suffered some kind of apocalypse a hundred-ish years ago. Our elites are keeping it quiet, and have altered the records, but they haven’t been able to destroy all the buildings of the lost world. Their cover story is that technology and wealth level haven’t regressed or anything, those kinds of buildings have just “gone out of style”.