Fossil water or paleowater is an ancient body of water that has been contained in some undisturbed space, typically groundwater in an aquifer, for millennia. Other types of fossil water can include subglacial lakes, such as Antarctica's Lake Vostok, and even ancient water on other planets. UNESCO defines fossil groundwater as Determining the time since water infiltrated usually involves analyzing isotopic signatures. Determining "fossil" status—whether or not that particular water has occupied that particular space since the distant past—involves modeling the flow, recharge, and losses of aquifers, which can involve significant uncertainty. Some aquifers are hundreds of meters deep and underlie vast areas of land. Research techniques in the field are developing quickly and the scientific knowledge base is growing. In the cases of many aquifers, research is lacking or disputed as to the age of the water and the behavior of the water inside the aquifer. Renewability Large, prolific aquifers (notably the Nubian Sandstone Aquifer System and the Ogallala Aquifer) containing fossil water are of significant socio-economic value. Fossil water is extracted from these aquifers for many human purposes, notably, agriculture, industry, and consumption. In arid regions, some aquifers containing available and usable water receive little to no significant recharge, effectively making groundwater in those aquifers a non-renewable resource. Extraction rates greater than recharge rates result in lowering of the water table and can lead to groundwater depletion. Extraction of non-renewable groundwater resources is referred to as groundwater "mining" because of their finite nature. General geology Aquifers are typically composed of semi-porous rock or unconsolidated material whose pore space has been filled with water. In the relatively rare cases of confined aquifers, an impermeable geologic layer (e.g. clay or calcrete) encloses an aquifer, isolating the water within, sometimes for millennia. More commonly, fossil water is found in arid or semi-arid regions where the climate was significantly more humid in recent geologic history. In some semi-arid regions, the majority of precipitation evaporates before it can infiltrate and result in any significant aquifer recharge. Most fossil groundwater has been estimated to have originally infiltrated within the Holocene and Pleistocene (10,000–40,000 years ago). Some fossil groundwater is associated with the melting of ice in the time since the last glacial maximum. Dating of groundwater relies on measuring concentrations of certain stable isotopes, including (tritium) and ("heavy" oxygen), and comparing values with known concentrations of the geologic past. Fossil water can potentially dissolve