Fossil water or paleowater is groundwater

Groundwater

Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of rock formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock...

 that has remained sealed in an aquifer

Aquifer

An aquifer is a wet underground layer of water-bearing permeable rock or unconsolidated materials from which groundwater can be usefully extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology...

 for a long period of time. Water can rest underground in "fossil aquifers" for thousands or even millions of years. When changes in the surrounding geology

Geology

Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...

 seal the aquifer off from further replenishing from precipitation, the water becomes trapped within, and is known as fossil water.

The Nubian Sandstone Aquifer System

Nubian Sandstone Aquifer System

The Nubian Sandstone Aquifer System is the world’s largest known fossil water aquifer system. It is located underground in the Eastern end of the Sahara Desert and spans the political boundaries of four countries in north-eastern Africa...

 is among the most notable of fossil water reserves. Fossil aquifers also exist in the Sahara

Sahara

The Sahara is the world's second largest desert, after Antarctica. At over , it covers most of Northern Africa, making it almost as large as Europe or the United States. The Sahara stretches from the Red Sea, including parts of the Mediterranean coasts, to the outskirts of the Atlantic Ocean...

, the Kalahari

Kalahari Desert

The Kalahari Desert is a large semi-arid sandy savannah in Southern Africa extending , covering much of Botswana and parts of Namibia and South Africa, as semi-desert, with huge tracts of excellent grazing after good rains. The Kalahari supports more animals and plants than a true desert...

, and the Ogallala

Ogallala Aquifer

The Ogallala Aquifer, also known as the High Plains Aquifer, is a vast yet shallow underground water table aquifer located beneath the Great Plains in the United States...

 underlying the US Great Plains. A further potential store of ancient water is Lake Vostok

Lake Vostok

Lake Vostok is the largest of more than 140 subglacial lakes found under the surface of Antarctica. The overlying ice provides a continuous paleoclimatic record of 400,000 years, although the lake water itself may have been isolated for 15 to 25 million years. The lake is named after the...

, a subglacial lake

Subglacial lake

A subglacial lake is a lake under a glacier, typically an ice cap or ice sheet. There are many such lakes, with Lake Vostok in Antarctica being by far the largest known at present.-Characteristics:...

 in Antarctica.

Fossil water is a non-renewable resource

Non-renewable resource

A non-renewable resource is a natural resource which cannot be produced, grown, generated, or used on a scale which can sustain its consumption rate, once depleted there is no more available for future needs. Also considered non-renewable are resources that are consumed much faster than nature...

. Whereas most aquifers are naturally replenished by infiltration of water from precipitation

Precipitation (meteorology)

In meteorology, precipitation In meteorology, precipitation In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation...

, fossil aquifers get very little recharge. The extraction of water from such non-replenishing groundwater

Groundwater

Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of rock formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock...

 reserves (known as low safe-yield reserves) is known in hydrology

Hydrology

Hydrology is the study of the movement, distribution, and quality of water on Earth and other planets, including the hydrologic cycle, water resources and environmental watershed sustainability...

 as "water mining

Mining

Mining is the extraction of valuable minerals or other geological materials from the earth, from an ore body, vein or seam. The term also includes the removal of soil. Materials recovered by mining include base metals, precious metals, iron, uranium, coal, diamonds, limestone, oil shale, rock...

". If water is pumped from a well at a withdrawal rate that exceeds the natural recharge rate (which is very low or zero for a fossil aquifer), the water table

Water table

The water table is the level at which the submarine pressure is far from atmospheric pressure. It may be conveniently visualized as the 'surface' of the subsurface materials that are saturated with groundwater in a given vicinity. However, saturated conditions may extend above the water table as...

 drops, forming a depression in the water levels around the well. Water mining has been blamed for contributing to rising sea levels.

Climatic effects of aquifer depletion

Aquifer drawdown

Drawdown

Drawdown has four distinct meanings:* Drawdown , a lowering of a reservoir or a change in hydraulic head in an aquifer, typically due to pumping a well.* Drawdown , decline in the value of an investment, below its all-time high....

 or overdrafting

Overdrafting

Overdrafting is the process of extracting groundwater beyond the safe yield or equilibrium yield of the aquifer.Since every groundwater basin recharges at a different rate depending upon precipitation, vegetative cover and soil conservation practises, the quantity of groundwater that can be safely...

 and the pumping of fossil water

Fossil water

Fossil water or paleowater is groundwater that has remained sealed in an aquifer for a long period of time. Water can rest underground in "fossil aquifers" for thousands or even millions of years...

 increases the total amount of water in the hydrosphere

Hydrosphere

A hydrosphere in physical geography describes the combined mass of water found on, under, and over the surface of a planet....

, and may be responsible for up to one quarter of the Earth's total sea level rise since the beginning of the 20th century.