Groundwater is a type of water located under the planet's surface. It is formed on top of dense rock or soil layers inside the ground.

Groundwater is often of a higher quality than surface water, because it is naturally filtered as it passes through soil and rock. The Finnish groundwater supply is five times larger than the amount of water found on the nation's surface. However, only 10 % of this water supply is currently used.

Groundwater is mostly used as water for drinking and domestic use. The largest and best quality groundwater supplies are found in eskers that comprise of sand and gravel as well as in gravel ridges such as the Salpausselkä ridges. The groundwater in these areas is so pure that it can be drunk without filtering. 

The quality of groundwater is damaged by sand and gravel collection from eskers and ridges, as well as by the excessive use of agricultural fertilizers and road salt.