The trans-Saharan exchange in which West African gold flowed north across the desert and Saharan and northern salt flowed south — the commercial engine that built the empires of Ghana, Mali, and Songhai and made cities like Timbuktu rich.
Two of the dullest substances in any modern kitchen once moved in opposite directions across the largest desert on Earth, and where each was scarce, salt could be worth nearly its weight in gold. The trade that carried them helped finance some of the richest empires the medieval world ever saw.