An ocean of sand 3,000 miles long and 1,000 miles wide where man and beast struggle to survive - that is Sahara. Across this empty land the salt caravans of the Tuaregs follow their age-old routes. From the salt mines of Bilma to the markets of Agades, more than 300 camels pace out 500 unmarked miles and then return - an incredible journey without end. An NBC-News Special