ISIS has conquered the largest dam in Iraq, Mosul Dam early August 2014. Not much attention has been paid to the event. However, having control over the largest dam in Iraq, is equivalent to the possession of a weapon of mass destruction. A deliberate breach of the dam would lead to a flood wave of immense proportions.

The dam is located at less than 50 kilometers upstream of the city of Mosul, a city of 1.8 million people. The dam is over 100 meters high and the length of the reservoir behind the dam is about 40 kilometers long. In case of a breach, large parts of the city of Mosul would be flooded by 20 meters of water within 2 hours after the breach; 300 kilometers further downstream, the capital of Baghdad could still be inundated with 5 meters of water. Hundred thousands of people would lose their lives.

Unfortunately, ISIS’ control over the Mosul Dam is not the only cause for concern. The dam was built in the 1980s on top of gypsum, which dissolves when it comes into contact with water. 'In terms of internal erosion potential of the foundation, Mosul Dam is the most dangerous dam in the world,' the United States' Army Corps of Engineers concluded in September 2006. 'If a small problem at Mosul Dam occurs, failure is likely.' Apart from this safety threat, since last weekend the Mosul Dam has become a severe security threat as well.

Deliberate attacks on flood defences are not well known. Some people would call these ‘unknown unknowns’. However, in the Second World War physical attacks on flood defences with explosives occurred regularly. Pilots from the Royal Air Force were called the dambusters. ‘Known unknowns’ is therefore a better term. Let us hope that an undesired failure, deliberate or undeliberate, of Mosul dam will not end up as a ‘known known’.

The authors both work at TU Delft Safety and Security Institute, which cooperates with Leiden University's Centre for Terrorism and Counterterrorism.