Apparently it's built on too much gypsum which is easily washed away. If too much is washed away the dam will sink and then fall apart. To counter this they've been pumping cement into the voids for years.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.
continued... http://www.newyorker.com/magazine/2017/ ... -than-isis