We're taking you down the river for today's Geo Quiz. We're looking for a tributary of the Amazon River in Brazil.

This tributary is a major river in its own right. It starts in the central Brazilian state of Mato Grosso. That's where six smaller rivers flow into each other to form a bigger body of water.

That's the one that then flows north toward the Amazon. The tributary meets the Amazon in the northern state of Para'. Now, the region where this tributary starts is in the news today.

Researchers have un-covered traces of ancient urban settlements underneath the jungle there. We're talking towns with grid-layouts and winding canoe canals.

These so-called "lost cities" existed before Columbus ever set foot in the New World. We tell you more in a few minutes.

First you have to name that Amazon tributary...