“FIRE or flood” goes an old saying about the turbulent history of Iraq; and just now flood is the one to fear. American officials are ringing alarm bells over a potentially catastrophic collapse of the country’s biggest dam, 50km (30 miles) upstream from Mosul, Iraq’s second city, currently held by Islamic State (IS)

“What we do know is this—if this dam were in the United States, we would have drained the lake behind it,” said Lieutenant-General Sean MacFarland, who commands American forces in Iraq, to reporters last month in Baghdad. Sensors installed by American army engineers in December show widening fissures in the fragile gypsum base underneath the dam, though no one can predict when a breach might occur.

Iraqi authorities have downplayed the danger, in part because they have not had the money to fix it. But after repeated warnings from American experts and an infusion of $200m from the World Bank for urgent repairs, the Iraqi government has now contracted with the Trevi Group, an Italian firm, which it hopes will offer a more advanced and permanent method of plugging cavities in the stone base than the constant maintenance it has required for the past 30 years. That maintenance came to an abrupt halt after IS seized the dam in August 2014, and has continued only intermittently after it was seized back three weeks later. Essential equipment went missing then, and half its staff decided not to return to work.

One study says that if the dam collapses, Mosul would be submerged within hours. Another warns that half a million Iraqis could be killed by floodwaters, and more than a million forced from their homes. Disease and looting as the floodwaters raced through Baiji, Tikrit, Samarra and even parts of Baghdad would complete that dreadful scenario.

The dam was built by an Italian-German consortium and started operating in 1986. Because of the high proportion of gypsum in the area, the construction included a grouting tunnel to allow almost constant injection of cement and drilling mud into crevices in the base that are widened by the water flowing through them. America’s Army Corps of Engineers warned in 2005 that the “extraordinary engineering measures” needed to maintain its structural integrity made the structure potentially the most dangerous in the world. But taking the dam out of commission is not an attractive option. Emptying the reservoir would leave Iraqis seriously short of drinking and unpolluted irrigation water in the summer.

What is clear, according to Iraqi military leaders and engineers, is that if the Mosul dam collapses, the city below it would be flooded so quickly that it would not stand much of a chance. “Downstream we can do many things” in co-operation with the UN and other Iraqi ministries, says Mehdi Rasheed of Iraq’s water ministry in Baghdad. “In the current situation that is not possible for the people in Mosul city.” The Italians had better get moving.