and absorb a number of ions from its host rock. Salinity in groundwater can be higher than seawater. In some cases, some form of treatment is required to make these waters suitable for human use. Saline fossil aquifers can also store significant quantities of oil and natural gas. Notable bodies of fossil water Ogallala Aquifer The Ogallala or High Plains Aquifer sits under 450,000 km2 of 8 states of the United States of America. It is one of the largest freshwater deposits in the world. The aquifer is composed of unconsolidated alluvial deposits. Groundwater in this aquifer has been dated to have been deposited in the humid time following the last glacial maximum. In much of the aquifer's area, an impermeable layer of calcrete prevents precipitation from infiltrating. In other regions of the aquifer, some relatively small rates of recharge have been measured. The aquifer supplies water for the many people who live above it and for widespread agricultural uses. In many areas, the water table has dropped drastically due to heavy extraction. Depletion rates are not stabilizing; in fact, they have been increasing in recent decades. Nubian Sandstone Aquifer System The Nubian Sandstone Aquifer System is located in northeastern Africa, under the nations of Sudan, Libya, Egypt, and Chad, covering about 2,000,000 km2. It is largely composed of many hydraulically interconnected sandstone aquifers. Some parts of the system are considered to be confined, if somewhat leaky, due to impermeable layers such as marine shales. The water was deposited between 4,000 and 20,000 years ago, varying by specific locality. The water in the Nubian Sandstone Aquifer System is of high importance to the people living above it, and has been for millennia. In modern times, as demand increases, avoiding rapid depletion and international conflict will depend on careful cross-boundary monitoring and planning. Libya and Egypt are currently planning development projects to withdraw significant amounts of the aquifer's fossil water for use. Other fossil aquifers have been identified throughout Northern Africa as well. Kalahari Desert aquifers The Kalahari Desert is in central southern Africa (Botswana, Namibia, and South Africa). Geology of the area includes significant karst formations. Most of the precipitation in the region evaporates before it can contribute to significant recharge of the aquifers below. Whether or not the region's aquifers receive any significant recharge has long been the subject of debate and research. In the northern region of the