Don’t build a dam on a gypsum foundation. Gypsum dissolves when it comes into contact with water. Nonetheless, that’s exactly what the all-wise, all-powerful Saddam Hussein did in the early 1980’s. The WaPo reports that “the largest dam in Iraq is in serious danger of an imminent collapse that could unleash a trillion-gallon wave of water.” The lives of 500,000 people down-river in Mosul and parts of Baghdad are now at risk. The cost of repairs: between $1 billion and $10 billion.