Fossil water

Fossil water or palaeowater is an ancient body of water that has been kept in an undisturbed place. Usually it is groundwater in an aquifer. It may stay underground for millions of years. Other types of fossil water can include lakes under ice, such as Antarctica's Lake Vostok. The term may be used to describe ancient water on other planets.
UNESCO defines "fossil groundwater" as
"water that infiltrated usually millennia ago and often under climatic conditions different from the present, and that has been stored underground since that time".
Estimating the time since water infiltrated is done using isotopes. 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. For many aquifers, research is lacking or disputed as to the age of the water and the behaviour of the water inside the aquifer.