There is also good evidence that there were extensive agricultural urban civilizations around and along the Mississippi. But they were so thoroughly destroyed (by the Little Ice Age, drought, and disease) that by the time European settlers got to those territories, even the descendant peoples had no memory of them. Only a handful of early explorers recorded them, and the principal archaeological traces are earth mounds (some rather large) built in their cities. For quite some time the &quot;Mound Builders&quot; were thought to be a different people entirely from the present native peoples.