Groundwater
Groundwater is water that fills the pores and cracks in soil and rock beneath the land surface, below the water table.
Definition
Groundwater is water that has soaked into the ground and fills the tiny spaces between soil grains and cracks in rock in the saturated zone below the water table, the surface it feeds by supplying wells, springs, streams, and wetlands. It is recharged by infiltrating precipitation and moves slowly downhill through porous rock according to Darcy's law, $Q = -KA\frac{dh}{dl}$, where hydraulic conductivity $K$ can span twelve orders of magnitude from clay to gravel, and it supplies drinking water for about half the United States population, making it the largest store of liquid fresh water on Earth. Residence times range from days in shallow alluvium to over a million years in deep basins, and over-pumping can cause drawdown cones, land subsidence, saltwater intrusion, and depletion of streams that depend on it for baseflow.
Example
Dig a hole at the beach and it fills with water rising to the level of the local water table, and many rural North Carolina homes draw water from wells drilled into fractured Piedmont bedrock while cities like Greenville and Wilmington rely on thick sandy Coastal Plain aquifers. The Ogallala Aquifer beneath the High Plains is being pumped far faster than it recharges, with water levels dropping over $30$ meters in parts of Kansas and Texas, and decades of pumping from the Castle Hayne and Black Creek aquifers in eastern North Carolina lowered heads enough that the state created capacity use areas to restrict withdrawals.
Key Insight
Groundwater is not an underground river or lake but water soaked into rock like a sponge, filling spaces too small to see, and it moves slowly, often less than a meter per day, so a pollutant spilled today may not reach a well for years and can take decades to clean up. Groundwater and surface water are really one connected system: rivers keep flowing through droughts because groundwater feeds them, so pumping a well near a stream eventually draws water from the stream itself.