We know that the vast majority of the Earth's surface is water. Unfortunately, 97.3% is salty, and only 2.7% is sweet. Of the latter, only a small fraction is available for domestic, agricultural, or industrial consumption, since the rest is in glaciers or underground layers that are too deep. An even smaller fraction is safe and drinkable. This text concentrates on fresh water, making brief allusions to the salty water of seas and oceans, because it is indispensable for life.
