Europeans sought the riches of Timbuktu for centuries. The wealth has long since disappeared, and a fragile culture is left behind. NPR's Alex Chadwick travels to the legendary city, and then far into the great Sahara for a glimpse of an ancient camel caravan culture. In the final report of a four-part NPR/National Geographic Radio Expedition to Mali, Chadwick descends into one of the biggest salt mines in the Sahara Desert, where West African miners still extract salt by hand, the way it's been done for centuries. Such mines have been the destination of camel caravans for at least a millennium. Anthropologist Wade Davis says the ancient mining and caravan traditions are threatened by modern times.