Much of the water on our planet is hidden beneath its surface. Since ancient times this water was called underground. So it is called now, although it would be more correct to characterize it as an inland or subsoil. Only  a relatively small fraction of this water comes to the surface in the  form of quiet keys, mountain streams or turbulent water fountains -  geysers. The main masses of it for thousands of years accumulate in invisible  pantries, squeezed between waterproof layers, until there is a free exit  somewhere or the bold intervention of a person will not release them.
The upper boundary of groundwater passes at various depths - from several meters to several tens and hundreds of meters. Below this boundary, almost all the pores and cracks in the rocks are  filled with water, except for certain sections of oil and gas fields  where the pores and cracks are occupied by oil and gas.
According  to calculations, in the upper five-kilometer thickness of the earth's  crust, 84.4 × 10 6 km3 of water are contained on the continents. Of these, 60 · 10 * 6 km3 is free (gravitational) water, capable of moving under the influence of gravity.
The entire earth's crust contains about 1.5 · 10 * 9 km3 of water, which is commensurate with the volume of the World Ocean.
Underground  waters are the only type of minerals whose reserves can be renewed  during operation, as they are a complex dynamic system that interacts  with the environment. Therefore, the groundwater reserves are subdivided into static (so-called century waters) and dynamic (renewable). The operational reserves of groundwater are determined by the flow, which can be continuously obtained per unit time.
The world's dynamic reserves of groundwater are 12,000 km3. These are underground waters of the so-called active water exchange  zone, which are drained by rivers and create a stable part of the river  flow.
Below the level of drainage of rivers, it is assumed that at least  9/10 of the total reserve of age-old groundwaters with very weak water  exchange activity is expected.
On the territory of Russia, as well as in Kazakhstan, huge lenses were found, equal in volume to several Aral seas. Extensive underground lenses of fresh water are found in the countries of Central Asia.
Ukraine has considerable resources of fresh groundwater. The fourth part of all water entering the country's water pipelines is  taken from underground sources, and in a number of regions - Lviv,  Kherson, Chernivtsi, etc., for drinking purposes only groundwater is  used.
At the expense of groundwater, almost 2/3 of Ukrainian cities are supplied with water. In many cities, for example, in Kyiv, both underground and surface water are used for water supply.