Much of the water in the universe is likely produced as a byproduct of star formation

A crucial resource that often is taken for granted, water affects all of our senses and many aspects of our lives. The ancient Greeks believed ice and water were two different things.

It wasn’t until 1784 that English physicist Henry Cavendish determined water’s composition through careful experimentation. By the 19th century, scientists had confirmed that it is composed of two elements: hydrogen and oxygen.

What’s more important than water? The neatly fused atoms of hydrogen and oxygen keep our 70-percent-water bodies alive, and clean as a bonus.

But where does it come from? What is its life cycle, what are the characteristics and effects of its forms (liquid, vapor, solid)? And perhaps most important, how is access to water changing as Earth ages and human life and biodiversity evolve?

Much of the water in the universe is likely produced as a byproduct of star formation— lots of gas and dust swirling around, condensino and forcing hydrogen and oxygen atoms together. Water has been detected in interstellar clouds within the Milky Way and probably exists in abundance in other galaxies, too, since hydrogen and oxygen are among the most abundant elements in the universe.

There are somewhere in the neighborhood of 326 million trillion gallons of water on Earth, constantly cycling between evaporation from the ocean, traveling through the air as vapor, and returning to the ground as rain before flowing back to the ocean.

98% of the planet’s water is in the oceans, making it unusable for drinking due to its salt content. The remaining 2% is freshwater, but much of that is locked in polar ice caps and glaciers. And so, less than 1% of the planet’s total water supply is found in lakes and rivers. That’s still thousands of trilions of gallons, but a fraction of the water avaliable

The oceans, too, are in perpetual motion, and not just due to the forces of wind and tides. Water has a life of its own, or so it seems: Warm water has more energy than cold water, because warmer molecules move around more. More active molecules also take up more space, so cold water is denser than warm water, which is why cold water sinks and warm water rises to the surface.

The process is called thermohaline circulation, sometimes referred to as the ocean conveyor belt because it moves ocean water around the planet. Ice, while colder, is less dense than liquid water—which is why icebergs float. Water molecules in ice are held in a rigid, six-sided geometric pattern by their hydrogen bonds.

This produces an open, porous structure, whereas the molecules in liquid water are more loosely bonded, allowing a greater number of molecules to occupy a given volume. 98% of the planet’s water is in the oceans, making it unusable for drinking due to its salt content. The remaining 2% is freshwater, but much of that is locked in polar ice caps and glaciers. And so, less than 1% of the planet’s total water supply is found in lakes and rivers.

That’s still thousands of trillions of gallons, but a fraction of all the water available. Communities naturally flourish around waterways—oceanic or fresh, even the practically unusable (example: Utah’s Great Salt Lake, inhabitable for only a scant few aquatic species, and used for extracting minerals). Lakes and rivers sustain life and provide natural beauty—the latter perhaps implied by the former.

Approximately 70 percent of freshwater is used in agriculture. Mesopotamia, the cradle of civilization, sat between the major rivers Tigris and Euphrates; ancient Egyptian society depended entirely upon the Nile. Cities such as London, Montreal, Paris, New York, Buenos Aires, Shanghai, Tokyo, Chicago, and Hong Kong owe their success in part to their easy accessibility via water and the resultant expansion of trade.

Availability of clean water is a major social and economic concern, especially as the human population grows. Access to safe drinking water has improved significantly in recent decades in almost every part of the world, but there is still a clear correlation between access to safe water and a nation’s per capita GDP. Currently, about a billion people around the world routinely drink unhealthy water, and more than five million of them die every year as a result.