Groundwater is water that soaks down through soil and rock and collects in spaces underground. When rain falls or snow melts, some water runs into streams and rivers, some evaporates back into the air, and some seeps down through the ground like water dripping through a sponge. This water keeps moving downward until it reaches layers of rock and soil that are completely saturated, creating vast underground reservoirs.

These underground water supplies are incredibly important. When you turn on a faucet in many homes, the water flowing out started as groundwater that was pumped up from wells drilled deep into the earth. About half of all Americans get their drinking water from groundwater, and farmers use it to irrigate crops in places where rain alone isn't enough.

Groundwater moves very slowly underground, sometimes taking hundreds of years to travel just a few miles. This slow movement means groundwater can stay clean and cold, filtered naturally by the soil and rock it passes through. But it also means that once groundwater gets polluted, it can take decades or even centuries to clean up. Some ancient groundwater has been underground for thousands of years, and when people pump it out faster than rain can replace it, those underground reserves can run dangerously low.