The sheer size of continents may be in some cases a direct source of aridity. Because most of the water in the atmosphere is ultimately derived from evaporation from the seas, there is often an aridity gradient in large continents: the land closer to the sea often receives a larger share of this oceanderived water and, as air moves inland, it gets depleted of moisture and precipitation drops. Thus, regions lying deep within a continent may become deserts simply because air currents reaching them have already traversed vast land distances and lost most of the moisture they originally carried. Continentality is a major factor driving arid conditions in the Monte Desert in South America (see Figure 1.4), in the central deserts of Australia, in the Great Basin in North America, and especially in the central East Asian deserts, the Taklimakan and the Gobi.