Century after century, camels and their drivers have traveled the sands between the fabled city of Timbuktu and the infamous salt mines of Taoudenni, hauling supplies from the proverbial end of the earth to an even farther-flung outpost, deep in Mali's slice of the Sahara. They return laden with tombstone-sized slabs of solid salt. While nearly all of the great trans-Saharan trade routes have disappeared, the Caravan of White Gold--so called because...