There's a mix of "conspiracy theories" surrounding this idea, from mud flood theories to Satan's little season and I'm sure there are others too.

Prior to 1800 is somewhat murky IMO, and the architecture doesn't match up with people who were riding around on dirt roads with horse and cart.

Couple that with many covered up pyramid structures around America and hieroglyphs found in the grand canyon which is largely off limits to the public, and it doesn't seem to crazy to question the validity of what we are told.