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Megacities of the world: a glimpse of how we'll live tomorrow

By 2050, 7 out of 10 people will live in megacities, offering the benefits of concentrated living but also some of the biggest public-works challenges in human history.

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Nearly 1,000 feet into the passageway, made up of adjacent rings each composed of concrete slabs weighing some 4 tons each, the air is thin. Oxygen roars in through a tube, providing relief for those working on the project's edge. Workers crawl along scaffolding, crouching under an Erector Set of tanks and pipes that pump out water and hurl the deep-earth's rock and mud to the surface.

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  • Graphic 2010
    (Rich Clabaugh/Staff)

  • Graphic 2025
    (Rich Clabaugh/Staff)

Their task is to prevent large portions of Mexico City, one of the world's most populous megacities, from catastrophic flooding. The area's growing population has placed demands on water supplies that are simply unsustainable. Its 20 million residents have laid down an urban jungle that obstructs water from naturally filtering into the ground.

Today, the city is sucking up water from the natural aquifers at twice the rate they are being replenished. The result: Mexico City is sinking, in some areas up to 16 inches a year, threatening its entire infrastructure. This includes the city's deteriorating drainage system, whose capacity has diminished by 30 percent since 1975 while the area's population has doubled.

"It's an alarming situation," says Felipe Arreguin, the technical general subdirector at Mexico's National Water Commission (Conagua), which is building the drainage tunnel. "We are taking [out] so much water, the city is sinking. What if an entire block were to go under?"

It nearly has. In 2007, a giant sinkhole swallowed a large swath of a busy street. At Revolution Monument, a water pipe installed over 75 years ago now stands near nearly 30 feet above ground. Given Mexico City's history as a "floating city" in the middle of a lake, it's no surprise that water is what vexes most urban planners here. When the Spaniards arrived to conquer the great Aztec Empire, the mode of transportation was not horses but canoes. Today, the city sits essentially on a bowl of pudding. Jose Miguel Guevara, the general coordinator for water supply and drainage projects at Conagua, calls this basin a giant "saucepan," with no natural exit for the torrential rains that fall each year. But these drainage problems and the corresponding threats of catastrophic flooding belie one of the great ironies of its urban plumbing. When it comes to water, the city is also facing the kind of shortages that plague the rest of the globe. Mexico City, which sits at an altitude of over 7,300 feet, must pump water up 3,000 feet to reach residents. Last year it had to ration water after one of the worst droughts in six decades. The drainage program includes plans for treatment plants to turn runoff into clean water for use by farmers.

These problems, and the enormously complex engineering and plumbing challenges they create, reveal a much larger global concern. Like Mexico City, megacities around the world must find ways to control runoff while providing clean water for millions of inhabitants. With 1.1 billion people – or 18 percent of the world's population – now lacking access to safe drinking water, according to the World Health Organization, governments of developing countries need the money and know-how to build massive public works.