To the north, the "Jol", the central limestone plateau of the southern Arab Peninsula, rises from the narrow coastal plain of the Gulf of Aden. From here, the caravans rode over the bare, dry plateau for a week before reaching the Wadi Hadhramawt with its cities of Al-Qatn, Shibam, Seiyun and Tarim. Running from east to west, the fertile canyon, up to six kilometres wide, and its tributary valleys form a completely independent ecosystem. Twice a year, around April and September, torrential rain may fall in the region. After strong rainfall on the "Jol", the water first gathers in the numerous side wadis (dry riverbeds) before running into the central Wadi Hadhramawt.