Most of the water we are aware of is in ponds, rivers, oceans, streams, lakes, puddles, and other places on top of the ground. What we don’t see is the water that soaks into the ground. Water that has "gone underground" is called groundwater.
If you traveled underground, you would eventually get deep enough to find that all the rock around you is saturated with water. You’d have entered the saturated zone! The height of water in the saturated zone is called the water table. The water table lies at varying depths beneath the surface. In dry places, the water table is deep, but in moist places, the water table is very shallow. When the water table is higher than the actual surface of the ground, surface water, including features such as streams, rivers, and lakes, cover the land.
The water in the saturated zone is called an aquifer. Not all aquifers are the same. Sometimes water becomes trapped a layer of rock that is sandwiched between two other impermeable layers that have no pores to let water through. This is called a confined aquifer. Other times, an aquifer forms above the saturated zone in places where water is trapped on top of solid, non-porous rocks that it cannot soak through. This is called a perched aquifer. Water gets into an aquifer from the surface. Typically, precipitation falling onto the Earth’s surface infiltrates into the ground and flows down to the water table.
An aquifer is a relatively harsh environment to live in. Not many living things can survive there. However, several types of microbes thrive in shallow aquifers. Additionally, many different types of living things, such as certain species of bats, fish, and microbes, are well adapted to living in caves.
Many people get the water they use in their house from wells that tap aquifers like a soda straw. The wells are drilled with a giant drill that is part of a big truck. Groundwater can become contaminated by human activity. Contaminated water has chemicals, such as pesticides or fertilizers, mixed in with the water. Wells need to be tested often to make sure there is no contamination.