On the morning of August 7, 2014, a team of fighters from the Islamic State, riding in pickup trucks and purloined American Humvees, swept out of the Iraqi village of Wana and headed for the Mosul Dam. Two months earlier, ISIS had captured Mosul, a city of nearly two million people, as part of a ruthless campaign to build a new caliphate in the Middle East. For an occupying force, the dam, twenty-five miles north of Mosul, was an appealing target: it regulates the flow of water to the city, and to millions of Iraqis who live along the Tigris. As the ISIS invaders approached, they could make out the dam’s four towers, standing over a wide, squat structure that looks like a brutalist mausoleum. Getting closer, they saw a retaining wall that spans the Tigris, rising three hundred and seventy feet from the riverbed and extending nearly two miles from embankment to embankment. Behind it, a reservoir eight miles long holds eleven billion cubic metres of water.

A group of Kurdish soldiers was stationed at the dam, and the ISIS fighters bombarded them from a distance and then moved in. When the battle was over, the area was nearly empty; most of the Iraqis who worked at the dam, a crew of nearly fifteen hundred, had fled. The fighters began to loot and destroy equipment. An ISIS propaganda video posted online shows a fighter carrying a flag across, and a man’s voice says, “The banner of unification flutters above the dam.”

The next day, Vice-President Joe Biden telephoned Masoud Barzani, the President of the Kurdish region, and urged him to retake the dam as quickly as possible. American officials feared that ISIS might try to blow it up, engulfing Mosul and a string of cities all the way to Baghdad in a colossal wave. Ten days later, after an intense struggle, Kurdish forces pushed out the ISIS fighters and took control of the dam.

But, in the months that followed, American officials inspected the dam and became concerned that it was on the brink of collapse. The problem wasn’t structural: the dam had been built to survive an aerial bombardment. (In fact, during the Gulf War, American jets bombed its generator, but the dam remained intact.) The problem, according to Azzam Alwash, an Iraqi-American civil engineer who has served as an adviser on the dam, is that “it’s just in the wrong place.” Completed in 1984, the dam sits on a foundation of soluble rock. To keep it stable, hundreds of employees have to work around the clock, pumping a cement mixture into the earth below. Without continuous maintenance, the rock beneath would wash away, causing the dam to sink and then break apart. But Iraq’s recent history has not been conducive to that kind of vigilance.

In October, Iraqi forces, backed by the United States, launched a sprawling military operation to retake Mosul, the largest city under ISIS control. The battle has sometimes been ferocious, with Iraqi soldiers facing suicide bombers, bombardments of chlorine gas, and legions of entrenched fighters. Although some Iraqi leaders predicted a quick success, it appears that the campaign to expel ISIS will be grinding and slow. And yet the biggest threat facing the people of northern Iraq may have nothing to do with who controls the streets.

In February, the U.S. Embassy in Baghdad issued a warning of the consequences of a breach in the dam. For a statement written by diplomats, it is extraordinarily blunt. “Mosul Dam faces a serious and unprecedented risk of catastrophic failure with little warning,” it said. Soon afterward, the United Nations released its own warning, predicting that “hundreds of thousands of people could be killed” if the dam failed. Iraq’s leaders, apparently fearful of public reaction, have refused to acknowledge the extent of the danger. But Alwash told me that nearly everyone outside the Iraqi government who has examined the dam believes that time is running out: in the spring, snowmelt flows into the Tigris, putting immense pressure on the retaining wall.

If the dam ruptured, it would likely cause a catastrophe of Biblical proportions, loosing a wave as high as a hundred feet that would roll down the Tigris, swallowing everything in its path for more than a hundred miles. Large parts of Mosul would be submerged in less than three hours. Along the riverbanks, towns and cities containing the heart of Iraq’s population would be flooded; in four days, a wave as high as sixteen feet would crash into Baghdad, a city of six million people. “If there is a breach in the dam, there will be no warning,” Alwash said. “It’s a nuclear bomb with an unpredictable fuse.”

Since civilization dawned in the Middle East, five and a half thousand years ago, the region’s politics and economy have centered on its two great rivers, the Tigris and the Euphrates. The rivers, which enter Iraq from the north and converge two hundred and fifty miles south of Baghdad, form an extraordinarily fertile valley in an otherwise dry part of the world. For centuries, populations flourished by tilling the rich alluvial soil left behind each spring by floodwaters receding from the plains between Baghdad and the Persian Gulf. But the rivers also wreaked havoc, delivering too much water or not enough, and the settlements on their banks lurched between periods of drought and flood.

In the nineteen-fifties, governments in the region moved to assert greater control over the rivers with aggressive programs of dam construction. Dams regularize the flow of water, discourage floods, and, by storing water in reservoirs, minimize the impact of droughts. They also give whoever controls them power over the flow of water downstream, rendering other countries vulnerable.

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In 1975, when both Syria and Turkey were completing dams on the Euphrates, and the reservoirs behind them began to fill, the river downstream dried up, forcing tens of thousands of Iraqi farmers to abandon their land. “You could walk across the Euphrates, it was so dry,” an Iraqi engineer who worked on the Mosul Dam told me. The same year, Turkey began surveying sites for another dam, just north of the border it shares with Iraq, on the Tigris River. Iraqi officials feared that, during the months or years when the new dam’s reservoir was being filled, many thousands of acres of farmland would have to be abandoned.

At the time, Saddam Hussein’s government was launching a hugely ambitious program of infrastructure development. The regime was awash in money; a previous government had nationalized the oil industry and renegotiated its relationships to the Western companies that had once controlled it. Saddam decided to build dams on both the Tigris and the Euphrates.

Western specialists began making surveys to find the most favorable site, but few places had the right topography for a reservoir: low-lying land, preferably surrounded by mountains. The geology presented even greater problems. Water in dam reservoirs creates tremendous pressure, and only solid rock can stop it from leaking underneath the dam. The surveys revealed a multilayer foundation of anhydrite, marl, and limestone, all interspersed with gypsum—which dissolves in contact with water. Dams built on this kind of rock are subject to a phenomenon called karstification, in which the foundation becomes shot through with voids and vacuums. According to former Iraqi officials who worked on the project, successive teams of geologists reached the same conclusion: no matter where they looked, the prevalence of gypsum would make maintaining a dam difficult.