"... 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 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. ..."

There's only one risk, that the dam will burst. Not the dam itself, but the surrounding foundations. They built it on Gypsum (has a tendency to dissolve in water), upkeep work (pumping grout into the voids) stopped for 1-18 months depending on who you ask.

"In the language of hydraulic engineering, the process eroding the foundation is known as “solutioning.” If that problem is not addressed, what happens next is “piping”: water begins to travel between the voids, moving horizontally beneath the dam."

"Schnittker told me, “Once piping begins, there is no going back. In twelve hours, the dam is gone.”"