Water is both mythic and real. It manages to be at once part of the mystery of life and part of the routine of life. We can use water to wash our dishes and our dogs and our cars without giving it a second thought, but few of us can resist simply standing and watching breakers crash on the beach. Water has all kinds of associations and connections, implications and suggestiveness. It also has an indispensable practicality.

Water is the most familiar substance in our lives. It is also unquestionably the most important substance in our lives. Water vapor is the insulation in our atmosphere that makes Earth a comfortable place for us to live. Water drives our weather and shapes our geography. Water is the lubricant that allows the continents themselves to move. Water is the secret ingredient of our fuel-hungry society. That new flat-screen TV, it turns out, needs not just a wall outlet and a cable connection but also its own water supply to get going. Who would have guessed?

Water is also the secret ingredient in the computer chips that make possible everything from MRI machines to Twitter accounts. Indeed, from blue jeans to iPhones, from Kleenex to basmati rice to the steel in your Toyota Prius, every product of modern life is awash in water. And water is, quite literally, everywhere. When you take a carton of milk from the refrigerator and set it on the table, within a minute or two the outside is covered in a film of condensation— water that has migrated almost instantly from the air of the kitchen to the cold surface of the milk carton.

Everything human beings do is, quite literally, a function of water, because every cell in our bodies is plumped full of it, and every cell is bathed in watery fluid. Blood is 83 percent water. Every heartbeat is mediated by chemicals in water; when we gaze at a starry night sky, the cells in our eyes execute all their seeing functions in water; thinking about water requires neurons filled with water.

Given that water is both the most familiar substance in our lives, and the most important substance in our lives, the really astonishing thing is that most of us don’t think of ourselves as having a relationship to water. It’s perfectly natural to talk about our relationship to our car or our relationship to food, our relationship to alcohol, or money, or to God. But water has achieved an invisibility in our lives that is only more remarkable given how central it is.

Back in 1999, a team of researchers recorded 289,000 toilet flushes of Americans in twelve cities, from Seattle to Tampa. The researchers used electronic water-flow sensors to record not just toilet flushes but every "water event" in each of 1,188 homes for four weeks. Although the study cost less than $1 million, it is considered so detailed and so pioneering that it hasn't been duplicated in the decade since; the U.S. Environmental Protection Agency continues to cite it as the definitive look at how Americans use water at home.

The researchers measured everything we do with water at home -- how many gallons a bath takes, how often the clothes washer runs, how much water the dishwasher uses, who has low-flow showerheads and who has regular, how many times we flush the toilet each day, and how many gallons of water each flush uses.

The study’s overall conclusion can be summed up in four words: We like to flush.

For Americans, flushing the toilet is the main way we use water. We use more water flushing toilets than bathing or cooking or washing our hands, our dishes, or our clothes. When we think about the big ways we use water, flushing the toilet doesn’t typically leap to mind. It’s one of those unnoticed parts of our daily water use -- our daily water-mark -- that turns out to be both startling and significant.

The largest single consumer of water in the United States, in fact, is virtually invisible. Every day, the nation’s power plants use 201 billion gallons of water in the course of generating electricity. That isn’t water used by hydroelectric plants -- it’s the water used by coal, gas, and nuclear power plants for cooling and to make steam.

Toilets and electric outlets may be stealthy consumers of water, but they at least serve vital functions. One of the largest daily consumers of water isn’t a use at all. One of every six gallons of water pumped into water mains by U.S. utilities simply leaks away, back into the ground.

Sixteen percent of the water disappears from the pipes before it makes it to a home or business or factory. Every six days, U.S. water utilities lose an entire day’s water. And that 16 percent U.S. loss rate isn’t too bad -- British utilities lose 19 percent of the water they pump; the French lose 26 percent. There is perhaps no better symbol of the golden age of water, of the carefree, almost cavalier, attitude that our abundance has fostered. We go to the trouble and expense to find city-size quantities of water, build dams, reservoirs, and tanks to store it and plants to treat it, then we pump it out to customers, only to let it dribble away before anyone can use it.

One of the hallmarks of the twentieth century, at least in the developed world, is that we have gradually been able to stop thinking about water. We use more of it than ever, we rely on it for purposes we not only never see but can hardly imagine, and we think about it not at all. It is a striking achievement. We used to build monuments -- even temples -- to water. The aqueducts of the Roman Empire are marvels of engineering and soaringly elegant design. They were plumbing presented as civic achievement and as a tribute to the water itself. Today, water has drifted so far from civic celebration that many people visit the Roman aqueducts without any sense at all that they moved water, or how.

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Many cities in the world are located where they are because of their proximity to water. For most of human history, in most settings, getting water was part of the daily routine; it was a constant part of our mental landscape. At the same time, humanity’s relationship to its water supply was wary, because water often made people sick. That’s why Poland Spring water was so popular in Boston and New York even a century ago -- it was safe.

One hundred years ago, with the dawn of bacteriology, two things happened. Cities started aggressively separating their freshwater supplies from their sewage disposal, something they had been surprisingly slow to do. (Philadelphia is just one of many cities whose sewage system, a hundred years ago, emptied into a river upstream of the city water supply intakes from the same river.)

And water utilities discovered that basic sand filters and chlorination could clean and disinfect water supplies, all but assuring their safety. In the decade from 1905 to 1915, as dozens of water systems around the country installed filters and chlorination systems, we went through a water revolution that profoundly improved human life forever.

Between 1900 and 1940, mortality rates in the United States fell 40 percent. How much did clean water matter? Harvard economist David Cutler and Stanford professor of medicine Grant Miller conducted a remarkable analysis, published in 2005, teasing out the impact of the new water treatment methods on the most dramatic reduction in death rates in U.S. history. By 1936, they conclude, simple filtration and chlorination of city water supplies reduced overall mortality in U.S. cities by 13 percent. Clean water cut child mortality in half.

Clean municipal water encouraged cities to grow, and it also encouraged the expansion of "mains water" during the twentieth century as the way most Americans got their water. (By 2005, only 14 percent of Americans still relied on wells or some other "self-supplied" water.) That first water revolution ushered in an era -- the one we think we still live in -- in which water was unlimited, free, and safe. And once it was unlimited, free, and safe, we could stop thinking about it.

The fact that it was unfailingly available "on demand" meant that we would use it more, even as we thought about it less.

Our very success with water ushered in not just a golden age of water, but a century-long era in which water became increasingly invisible. Our home water bills, which are less than half our monthly cable TV or cell phone bills, provide almost no insight into how much water we use, or how we use it -- even if we study them.

The new class of micropollutants we are beginning to hear about -- infinitesimal, almost molecular, traces of plastics, birth control pills, antidepressants -- have literally been invisible even to chemists until very recently; you certainly can’t tell if they’re in your water by looking at it or drinking it. The impact of those micropollutants on our health, if any, may remain invisible for years -- and may be almost impossible to predict or trace.

Even our emotional connections to water have become submerged and camouflaged -- the ease with which water enters and leaves our lives allows us an indifference to our water supply. We are utterly ignorant of our own water-mark, of the amount of water required to float us through the day, and we are utterly indifferent to the mark our daily life leaves on the water supply.

But the golden age of water is rapidly coming to an end. The last century has conditioned us to think that water is naturally abundant, safe, and cheap — that it should be, that it will be. We’re in for a rude shock.

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We are in the middle of a water crisis already, in the United States and around the world. The experts realize it (the Weather Channel already has a dedicated burning-orange logo for its drought reports), but even in areas with serious water problems, most people don’t seem to understand. We are entering a new era of water scarcity -- not just in traditionally dry or hard-pressed places like the U.S. Southwest and the Middle East, but in places we think of as water-wealthy, like Atlanta and Melbourne.

The world has 6.9 billion people. At least 1.1 billion of us don’t have access to clean, safe drinking water -- that’s one out of six people in the world. Another 1.8 billion people don’t have access to water in their homes or yard, but do have access within a kilometer. So at least 40 percent of the world either doesn’t have good access to water, or has to walk to get it.

In the next fifteen years, by 2025, the world will add 1.2 billion people. By 2050, we will add 2.4 billion people. So between now and forty years from now, more new people will join the total population than were alive worldwide in 1900. They will be thirsty.

And then there is the unpredictability of climate change. Water availability is intensely weather- and climate-dependent, in both the developed world and the developing world. At one point in 2008, during the years-long drought across the southeastern United States, 80 percent of the residents of North Carolina were living under water-use restrictions.

The Las Vegas area has 2 million residents and 36 million visitors a year, and its water source in January 2011 was lower than it had been in any January going back to 1965. At that time, Las Vegas had about 200,000 residents; today, on a typical day, there are twice that many tourists in town.

Beyond population and climate change, the other huge and growing pressure on water supplies is economic development. China and India are modernizing at a whirling pace, and together those two countries account for one out of three people in the world. Economic development requires rivers full of water, not just because people want more secure and more abundant water as their incomes improve but because modern factories and businesses use such huge volumes of water.

It is a mistake to think that big water issues are not manageable, however. One of the most startling, inspiring and least well-known examples involves the United States. The United States uses less water today than it did in 1980. Not in per capita terms, in absolute terms. Water use in the United States peaked in 1980, at 440 billion gallons a day for all purposes. Today, the country is using about 410 billion gallons of water a day.

That performance is amazing in many ways. Since 1980, the U.S. population has grown by 70 million people. And since 1980, the U.S. GDP in real terms has more than doubled. We use less water to create a $13 trillion economy today than we needed to create a $6 trillion economy then.

In fact, the most unsettling attitude we’ve begun to develop about water is a kind of disdain for the era we’ve just lived through. The very universal access that has been the core of our water philosophy for the last hundred years -- the provision of clean, dependable tap water that created the golden age of water -- that very principle has turned on its head.

The brilliant invisibility of our water system -- the sources of water unknown to the people who use it, the pipes buried under pavement, the treatment plants anonymous and tucked away, the water service itself so reliable that even the reliability is a kind of invisibility -- that invisibility has become the system’s most significant vulnerability.

That invisibility makes it difficult for people to understand the effort and money required to sustain a system that has been in place for decades, but has in fact been quietly corroding from decades of neglect. Why should I pay higher taxes just to replace some old water pipes? I’ll just drink bottled water if I don’t like what comes out of the tap. It is almost as if tap water is regarded not with respect and appreciation but with a hint of condescension, even contempt.

Of course, you can’t call Dasani if your house catches on fire. We are in danger of allowing ourselves to imagine that since we’ve got FedEx, we don’t also need the postal service. When universal, twenty-four-hour-a-day access to water starts to slip away, it becomes very hard to bring back. But sustaining it requires more than paying the monthly water bill. If we’re going to be ready for a new era of water, we need to reclaim water from our superficial sense of it, we need to reclaim it from the clichés. We need to rediscover its true value, and also the serious commitment required to provide it. It is one of the ironies of our relationship to water that the moment it becomes unavailable, the moment it really disappears -- that’s when water becomes most urgently visible.