Origin and quantity of groundwater on the earth
Most groundwater originates as meteoric water from precipitation in the form of rain or snow. If it is not lost by evaporation, transpiration or to stream runoff, water from these sources may infiltrate into the ground. Initial amounts of water from precipitation onto dry soil are held very tightly as a film on the surfaces and in the micro pores of soil particles in a belt of soil mixture. At intermediate levels, films of water cover the solid particles, but air is still present in the voids of the soil. This region is called unsaturated zone or zone of aeration, and the water present is vadose water. At lower depths and in presence of adequate amounts of water, all voids are filled to produce a zone of saturation, the upper level of which is the water table. Water present in a zone of saturation is called groundwater .
The porosity and structure of the ground determine the type of aquifer and underground circulation. groundwater may circulate and be stored in the entire geological stratum: this is the case in porous soils such as sand, sandstone and alluvium. It may circulate and be stored in fissures or faults in compact rocks that are not themselves permeable, like most of volcanic and metamorphic rocks. Water trickles through the rocks and circulates because of localized and dispersed fissures. Compact rocks of large fissures or caverns are typical of limestone.
Quantity in the earth
On the earth, approximately 3% of the total water is fresh water. Of this groundwater comprises 95%, surface water 3.5% and soil moisture 1.5%. Out of all the fresh water on the earth, only 0.36% is readily available to use (Leopold, 1974).
We are withdrawing water from underground aquifers at a faster rate that it can be replenished. Although immense, world's aquifers are not bottomless and in many areas water levels are sinking fast. The water in some aquifers is millennia old and lies beneath what are now some of the driest regions on Earth. Although people have drown water from from springs and wells since the earliest civilizations, in the past 50 years multiplying populations have needed more food and water and the rate of withdrawal has increased drammatically.
Punjab, India and Pakistan:
North China Plain:
Groundwater is also affected by water engineering: for decades and centuries, through improper disposal of wastes to the environment and subsurface areas many groundwater have become contaminated. Efforts to protect the quality and quantity of groundwater have been made by cooperation between all government agencies, industrial parties and researchers. 
Here you find a map of groundwater distribution on the earth .
Click here for definitions concerning groundwater, to learn more about its properties, its sources in Europe or the problem of its contamination.