Life on earth is dependent upon water. About 60% of the human body is water. Water has a number of properties that make it useful for living things, including its ability to act as a solvent, its high boiling point, and the fact that it is lighter as a solid (ice) than as a liquid. Over 70% of the earth's surface is covered with water but only 3% of this is the fresh water needed by nonmarine species. The majority of this fresh water is unavailable to living things because it is frozen in glaciers. A very small proportion is available, although the amount is more than sufficient to sustain life. The key issues for humans with regard to water are quantity and quality.