Pure salt crystallizes on the flats beside Lake Assal. At 509 feet below sea level, Lake Assal is the lowest place in Africa. From time immemorial, nomadic Afar tribesmen have come here with their camels to collect salt from deposits that are at least 109 feet thick. Lake Assal is fed by seawater from fissures deep underground. Consequently, the salt is a constantly renewing resource. The water contains 348 grams of dissolved sodium chloride per litre _ an astonishingly high concentration caused by evaporation in the scorching sun.