[ROLF of the HILL COUNTRY]

I think it's partly hearsay from fishermen, etc. AND remnants of knowledge from an earlier civilization.
The Piri Reis map; Plato's description of the “great western continent across the Atlantic;” other similar writings from various sources; anachronistic artifacts from antiquity, all point to the presence of a civilization comparable to that of Rome that flourished before known history.

[gleeaikin]

“a civilization comparable to that of Rome that flourished before known history.”

An will continue not to be known until archeology takes up serious underwater coastal and riverine exploration to find settlements along the ancient (much lower) shorelines.