ad-Dahna Desert is the central division of the Arabian Desert. It is a corridor of sandy terrain forming a bow-like shape that connects an-Nafud desert in the north to Rub' al-Khali desert in the south. Its length is more than 1000 km siding Twaik Mountains from the east and does not exceed 80 km in width. It is also considered the geographical margin separating Al-Ahsa Province from Najd.
Al-Dahna desert is formed of high sand dunes spreading horizontally which are called veins (ar: عروق), mostly red in color since it contains Iron Oxides.
Beneath the harsh deserts of Saudi Arabia lie dark chambers and complex mazes filled Crystalline structures, stalactites and stalagmites. The limestone floor of the Summan plateau, a karst area to the east of the Dahna sands, is riddled with such caves, known locally as Dahls. Some have tiny entrances which open into caves, others lead into a maze of passages which can be several kilometers long. Essential to the formation of these underground cavities is the action of water percolating through the soluble rocks. The process is slow. Rainwater absorbs fractional amounts of Carbon dioxide from the air as it falls, making a weak carbonic acid solution that eats into the limestone and eventually forms hollows and channels Local bedouin have always know these caves and some were used as water supplies. First systematically studied in the 1981,and currently being explored and reported by the Saudi Geological survey.
This article incorporates text from the public domain 1907 edition of The Nuttall Encyclopædia.