If breached, it could unleash a 180-foot-high wave down the Tigris River Basin and drown more than half a million people, with floodwaters reaching as far as the Iraqi capital, about 280 miles to the south.
The collapse of the Mosul Dam would be catastrophic for Iraq.
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The dam’s structural problems became evident as soon as the reservoir behind it was filled in 1985. It is built on layers of clay and gypsum, a soft mineral that dissolves when it comes into contact with water, and the dam immediately began seeping. Since then, about 100,000 tons of grout have been poured into the structure to prevent it from collapsing.
But even this stopgap measure has been disrupted by the Islamic State, which briefly seized the dam in the summer of 2014. The militants still hold the nearby city of Mosul, their de facto capital in Iraq. Political wrangling and a financial crisis in Iraq also are complicating repair work.
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When the Islamic State took control of the dam, a rigid daily routine of pouring grout into the structure to keep it from collapsing was missed for six weeks, and logistical issues have plagued the process ever since.
Meanwhile, a government decision to deprive Islamic State-held Mosul of electricity by blocking the flow of water put additional pressure on the dam as water levels rose.
