Groundwater can be found almost everywhere on earth. We just can't see it. Above-ground lakes and rivers only account for a tiny proportion of liquidfresh water. Some 99% of the rest is subterranean.  

Groundwater often lies just a few meters below the surface. But, in some cases, it's been sitting for millions of years at depths of up to 2 kilometers (6,561 feet) in gigantic reservoirs and underground networks that span several countries and hundreds of thousands of square kilometers.

There is so much groundwater that it would cover the entire terrestrial surface of the earth with a 180-meter-thick layer.

And this hidden water is the liquid link between many ecosystems such as rivers, lakes, lagoons, natural springs, and between wet and dry areas. It plays a key role in the water cycle and consequently for all life on the planet. 

Why is groundwater so important?

Experts sometimes describe groundwater as mankind's savings account. Groundwater supplies about half of the world's drinking water, 40% of the water used for agriculture and 30% of industrial water needs. 

It is often the only source of water, particularly in arid regions. Oases and natural springs that support plants and animals in such areas are fed solely by this out-of-sight resource. 

But in some parts of the world, people face severe water scarcity because groundwater is being depleted faster than it is naturally replenished. 

And, as climate change intensifies, some areas could become more dependent on groundwater as rain becomes less frequent but falls in heavier bursts.

Pollution is another major threat to supplies and the ecosystems and billions of people dependent on them. This can occur when chemicals, untreated industrial waste, contamination from agricultural fertilizers and leaky landfill sites or urban wastewater seep into the earth's depths, reaching the groundwater.

While the United Nations says supplies in most parts of the world are still safe to use, there's much regional variation in quality and quantity. 

In Spain, Iran, China and the USA, for example, underground water reservoirs are sinking by 2 meters a year in some places, and wells are being drilled deeper and deeper. Yet sub-Saharan Africa still has plenty of groundwater. There is enormous potential to tap into vast reserves below the surface and supply people and farms, especially in areas with frequent droughts. 

How does groundwater replenish?

Groundwater is replenished primarily by rain or melting snow. Water seeps into the ground, finding crevices and cavities and accumulating in reservoirs. The water table separates the saturated underground area and the unsaturated land above it. 

The amount of rainfall, soil conditions and the time of year all play a role in whether groundwater can replenish.

In some European regions, for instance, it tends to rain more in summer. But high summer temperatures cause more water evaporation, while plants are even thirstier. This means that very little water reaches the subsoil. In winter, when plants require less water and hardly any evaporates, groundwater reservoirs are usually replenished.

In areas where this underground resource has been over-exploited, some countries have managed to replenish supplies by implementing policies that reduce demand for groundwater, particularly from agriculture, and intentionally recharging it with surface water. 

This article was adapted from German by Alistair Walsh.