By Helen Colella
When looking at pictures of the Earth taken from outer space, it’s obvious why it’s often referred to as the Blue Planet. Seventy percent of the Earth's surface is water-covered, with the oceans holding about 96.5 percent of that. Water also exists in the air as vapor, in rivers and lakes, in icecaps and glaciers, in the ground as soil moisture and aquifers, and in humans.
