Water

Water is a source of energy and an indispensable element of life. All life on Earth depends on water to survive. The vapour of ocean currents helps maintain the planet's surface temperature within its livable limits. Water also forms a part of almost everything on Earth.

Water exhibits a unique set of chemical and physical characteristics. These unusual properties include its ability to exist as a solid, liquid or vapour at the temperatures and pressures found at and near the Earth's surface. This ability enables water to circulate through the atmosphere, ocean and solid earth, forming endless cycles that are known collectively as the hydrologic cycle, or water cycle. Water vapour plays a key role in the water cycle while acting as a small but critical atmospheric constituent. Thus, water is a unique substance that is essential to life and a key ingredient of our weather and climate system.

Source

Centre Street bridge, destroyed by the 1915 flood, Calgary

The conditions that were necessary for water to appear on the planet are so unique as to be highly improbable: oxygen and hydrogen had to stay in the atmosphere and meet; and the Earth had to be the right size and distance from the sun. When the first fluid fell from the sky, it was water. (On Venus, clouds rain concentrated sulphuric acid, one of the most corrosive fluids in the universe.)