The water-supply of the oasis is derived from an underground bed of sandstone, 55 metres thick, underlying a dense impervious red clay 45 metres in thickness; the upper part of the latter is conspicuous throughout the oasis, underlying the alluvium and forming the base of the surrounding escarpments in many localities. Below the water-bearing sandstone lies a black clay, never yet penetrated by the boring rods; it is probable that other water-tables exist below and such would be invaluable for the irrigation of those parts of the oasis where the present supply is unsatisfactory. There seem to be no natural springs extant at the present day, the whole of the water-supply being through boreholes, both ancient and modern. The old wells, known as _ain_, _aiyûn_, appear to be mostly of early Egyptian and Roman construction, and number over 400; exactly similar wells have been sunk by boring plant during the last few decades and are called _bîr_, _biyâr_; there are over 160 of these; all are true artesian wells. At the present day the method in vogue is as follows:--a two metre square timbered shaft is sunk by hand to the base of the red clay and within this is built up a watertight wooden pipe, 35 cm. in diameter, made of ‘sunt’ (a species of thorny acacia), the surrounding space being packed with clay. Sinking is continued in the sandstone with the boring machine until a satisfactory flow of water is obtained. Many of the older wells in the oasis have become choked up, and although some have been successfully cleaned out by the inhabitants, but the process is costly and laborious and frequently fails. The work is done by divers, a small but hardy class only found in Dakhla and Farafra.