"... 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.'"
From "A Bigger Problem Than ISIS?/The Mosul Dam is failing. A breach would cause a colossal wave that could kill as many as a million and a half people," by Dexter Filkins in The New Yorker. The dam "sits on a foundation of soluble rock" and requires "hundreds of employees... to work around the clock, pumping a cement mixture into the earth below."
ADDED: "Today, a stone memorial on top of the dam commemorates nineteen Chinese nationals who died during its construction; the memorial, inscribed in English and Chinese but not in Arabic, does not give the cause of their deaths. Alwash, the Iraqi-American hydrological engineer, told me that, in Iraq, when laborers fell into wet cement during large infrastructure projects, it was common for the work to carry on. 'When you’re laying that much cement on a dam, you can’t stop,' Alwash said."