But ocean water is salty, and not easily converted to freshwater. The amount of freshwater available for human use is only a small fraction of the amount of water found in oceans or locked away in polar ice caps. Today, 1.1 billion people still lack access to safe drinking water For the most part, underground water flows in the spaces between sand and gravel or in the cracks of rocks.
Aquifers form in the loose soil lying directly above a layer of clay or less permeable rock.
In certain limestones, groundwater can accumulate and flow in large caverns and conduits. Aquifers are replenished by rain and surface water percolating downward through the soil. Underground water is in motion most of the time, flowing slowly from recharge areas until it discharges into a spring, stream, lake, wetlands or ocean. Groundwater often follows the course of rivers or lies underneath marshes and swamps, keeping rivers from drying up and protecting vegetation when rain is scarce. The upward and downward movement of water through the ground has also a filtering effect, accounting for the generally good quality of groundwater.