As the light fades on a blistering day, an Afar man digs for salt at Lake Assal.At 509 feet below sea level, Lake Assal is the lowest place in Africa. Nomadic Afar tribesmen come here with their camels to collect salt. Extremely high midday temperatures, which can surpass 1200 F, force them to arrive in the late afternoon and immediately set about collecting salt from deposits that are at least 109 feet thick. They work late into the night. The camel caravans leave again before daybreak the following day. Each animal carries between four and ten thirty-pound sacks according to its size and maturity. The salt is sold across the border in Ethiopia.