Actually, we can explain how ancient Indigenous civilizations managed to create such feats of engineering and architecture. Much in the same way we can explain how pre-Roman Europeans, ancient Egyptians, Babylonians, South and East Asians did the same.

The "ancient aliens" idiots seem to think there's no explanation merely because they are completely and totally ignorant or archaeology. They are not academics with opinions, just weak-minded fools who want to believe asinine theories.