Earth is known as "The Blue Planet" because of its massive oceans and other bodies of water. The University of Michigan website indicates that 71 percent of the Earth is covered by deep oceans. Views of Earth from space also demonstrate its vibrant blue appearance.
Another motive for this nickname is that Earth is the only planet in the solar system with such an abundant water supply. Availability of water is a primary factor scientists look for in trying to identify another habitable planet. Even on Mars, where studies of potential human existence have been greatest, there is little surface liquid.