Ground water comes from rain water, melting snow or water run-off provided by irrigation seeping into the ground.
Some of it will be taken up by plant roots. The rest will seep deeper into the ground, sometimes aided by cracks and crevices or empty spaces between rocks. It doesn’t actually flow as huge rivers underground; rather, it trickles between rocks.
It continues seeping deeper until it reaches a rocky level (or an impermeable layer) through which there are no spaces for it to penetrate further down. Here,i t starts to build up in an underground pool, and at this point, it’s called “ground water.”
An “aquifer” is a large underground area, porous but not empty, consisting of gravel, sand, fractured rocks, etc, which gets filled with ground-water all through it.
Ground water can be extracted artificially through wells, naturally through artesian wells, or can come out naturally through springs, either on dry land or into other water such as lakes and rivers.
Ground water can run out if more is taken out than is coming back in to replenish it, such as during dry spells. It can be polluted by pesticides, fertilizers, septic tanks, run-off from livestock, garbage dumps, fuel depots, etc.
At any one time, roughly 30% of the freshwater on earth is under the ground.