Ad-Dahna Desert passages which can be several kilometers long. Local Bedouin have always known these caves and some were used as water supplies. They were first systematically studied in the 1981, and later explored and reported by the Saudi Geological Survey. Ad-Dahna Desert The 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 1,000 km siding Twaik Mountains from the east and does not exceed 80 km in width. It is
Jabal Aja Protected Area Area because of the richness of its plant life and the presence of many endemic and relict plant species. The reserve offers a refuge to plants from the Mediterranean and Irano-Turanian Regions in an area that otherwise forms part of the Saharo-Arabian Region. About 355 species of plant have been recorded in the reserve, including the plants found in the adjoining desert area of An Nafud. The plains have deep sandy-loam soils and the dominant plant is "Haloxylon salicornicum", with the desert gourd being common and "Asphodelus tenuifolius" occurring in depressions. The annual plants "Stipa tortilis", "Picris cyanocarpa" and "Anthemis"