The Mandans were an agricultural-based tribe with cities on the Missouri River (not Mississippi) in western North Dakota. Lewis and Clark spent the first winter of their expedition with the Mandans. They were largely exterminated within a few years by European diseases especially small pox. What has provoked much interest and speculation about them is that they were farmers surrounded by tribes of hunters and that there were a significant number of fair-haired, blue-eyed individuals in the population.
Then there is the purported "runestone" dug up in Minnesota which has sparked controversy for years and debate as to whether it is real or a clever fake. The stone has old Norse writing on it.