Gauab As, Western Namibia, a cave in a megacrystalline hydrothermal Dolomite. The authors describe a cave in the semi-desert area of the Hakos Mountains, 100 km to the southwest of Windhoek, Namibia. The cave is significant due to the very unusual country rock , with which it is associated. It formed by dissolution of the dolomite core of a large quartz vein which is 800 m long and 200 m wide, developed in micaschist. The cave consists of a complex succession of large chambers, more or less overlapping each others, with walls generally consisting of quartz. In most instances the dolomite has been completely dissolved or occurs under the floor, concealed by dust and scree. Although the cave developed within a very small volume of carbonate, the total length reaches 695 m and the depth 85 m. The bottom is occupied by a pool, which is only temporarily filled with water and probably marks the position of a perched water-table. The cave formed in a perched phreatic environment during an undetermined period.