الربع الخالي
Wadi Hadhramaut and its tributaries have been inhabited since the Stone Age. Small mounds of flint chippings – debris from the manufacture of stone tools and weapons – and windblown dust can be found close to canyon walls. Further north and east are lines of Thamudic ‘triliths’ with a few surviving crude inscriptions. On the fringes of the Rub' al Khali north of Mahra, a seemingly ancient track leads – according to local legend – to the lost city of Ubar.