How Aquifers Store Water
Aquifers are made of porous materials like sandstone, gravel, limestone, and other types of rock. These materials have many tiny holes and spaces called pores. Water fills these pores and moves slowly through the ground. The rock acts like a sponge, holding water between its particles. Different types of rock hold different amounts of water depending on how many pores they have.
Types of Aquifers
There are two main types of aquifers. Unconfined aquifers are closer to the surface and are directly connected to groundwater from rain and snow. Confined aquifers are trapped between layers of rock or clay that prevent water from moving up or down easily. Confined aquifers often hold water under pressure, which can push water up through wells without needing a pump.
Water Movement in Aquifers
Water moves through aquifers very slowly, sometimes only a few feet per year. It flows from areas where the water level is higher to areas where it is lower, following the slope of the underground layers. This slow movement is important because it filters the water naturally as it passes through rocks and soil, making much of the groundwater clean and safe to drink.
Recharge and Depletion
Aquifers are recharged when water from rain and snow soaks into the ground and filters down through the soil and rock. However, in some areas, people use more water than the aquifer can replace. This causes the water table to drop over time. Some aquifers, called fossil aquifers, contain water that fell as rain thousands of years ago and receive little to no recharge today.
Human Use and Importance
Aquifers supply drinking water to billions of people and are essential for agriculture and industry. Wells are drilled into aquifers to bring water to the surface for human use. In many parts of the world, groundwater from aquifers is the primary source of fresh water, especially in areas with little rainfall or surface water sources. 
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