BAGHDAD — If breached, the Mosul Dam 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 dam that has been called the most dangerous in the world for the past decade would be catastrophic for Iraq.

And, recent assessments by the U.S. Army Corps of Engineers say it is at “significantly higher risk” of failing than previously thought.

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 grouting have been poured into the structure to prevent it from collapsing.

However, 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.