Why Control of a Terrifying Dam in Iraq Is Life or Death for Half Million People "Catastrophic" worst case scenario: 65-foot wall of water inundates whole city.

 -- There are conflicting reports out today about whether the extremist group ISIS has taken control over Iraq's largest and most dangerous dam, which Iraqi officials had previously said was safe under the protection of Kurdish forces.

ISIS, the Islamic State of Iraq and Syria, wrote on their website today that they are in control of the two-mile-wide Mosul Dam, echoing claims the group made over the weekend. Iraqi media reports and a Kurdish official have supported the claim. But late Wednesday and early today, two Iraqi government officials, one from the Ministry of Water Resources and the other familiar with the dam’s operations, told ABC News ISIS had not taken the dam and said that it is functioning as usual.

The question of control is a critical one for the millions of Iraqis who live downstream of the Mosul Dam all the way down the Tigris to Baghdad, because if the dam was taken over, ISIS would be in control of what effectively could be a major weapon of mass destruction – one that the U.S. military said in 2006 was, without the help of brutal jihadists, already “the most dangerous dam in the world.”

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It wouldn’t even have to be sabotaged to fail – if an extremist group took control and wanted the dam to break, they may be able to simply do nothing.

The gargantuan dam, built in the mid-1980s, was constructed on “a foundation of soluble soils that are continuously dissolving, resulting in the formation of cavities and voids underground that place the dam at risk for failure,” said an urgent letter sent from David Petraeus, then commanding general of the U.S. Army, and Ryan Crocker, then U.S. Ambassador to Iraq, to Iraqi Prime Minister Nouri al-Maliki in 2007.

The dam requires “extraordinary engineering measures” – namely constant grouting operations -- to fill in the holes and “maintain the structural integrity and operating capability of the dam,” according to a U.S. Army Corps of Engineers (USACE) report from the same year.

For 30 years –- and through several periods of violent conflict -- the Iraqi government has managed to keep the dam upright by continuously pumping in literally tons of grout like an industrial version of the little Dutch boy, as a geotechnical expert who worked on the dam put it.

But the U.S. says any failure of the dam could be “catastrophic.”

“[T]he most severe impact of a dam failure would be [for] the City of Mosul, located 50 kilometers [31 miles] downstream of the dam,” Petraeus’ and Crocker’s 2007 letter says. “Assuming a worse [sic] case scenario, an instantaneous failure of Mosul Dam filled to its maximum operating level could result in a flood wave over 20 meters [65 feet] deep at the City of Mosul, which would result in a significant loss of life and property.” Mosul alone is estimated to be home to more than 1.5 million people. Flood waters, albeit at a lower level, could reach all the way to Baghdad, more than 200 miles further down the Tigris, depending on the performance of another smaller dam further downriver.

A 2011 report written by a USACE official and published in Water Power magazine estimated failure “could lead to as many as 500,000 civilian deaths.”

The Water Power article states that Iraq is “fully aware of the challenges facing the ageing structure,” but as USACE civil engineer David Paul told the magazine at the time, “there is no precedence for what they are trying to achieve” in finding a more permanent solution to the dam’s problems than never-ending grouting – including the proposed use of an incredibly large “cutoff wall” to help mitigate the seepage. There are other measures that can be taken, such as keeping the reservoir levels lower than the maximum to reduce pressure on the dam; that was one of several recommendations the U.S. government made in 2007.

But none totally fix the problem and the geotechnical expert who spoke to ABC News said that he didn’t have reason to believe the dam is any better off today than it was when the USACE report was published in 2007.

That was also before a powerful jihadist group borne of the Syrian civil war began its deadly march across Iraq and reportedly up to Mosul Dam’s doorstep. Like today, earlier this week there were conflicting reports about whether ISIS had taken control of the dam during a previous 24-hour offensive in the area.

Tuesday the Iraqi Ministry of Water Resources circulated a statement saying the dam was not under ISIS control but has been protected by Kurdish peshmerga troops. The government department reiterated the claim earlier today.

A second Iraqi official involved with the dam’s operations said Wednesday that grouting supplies were safe and there was plenty in store.

“Grouting is still ongoing and never stopped,” said the official, who asked his name not be used because he was not authorized to speak to the media.

But what if ISIS does eventually overtake the dam? Or what if it already has?

State Department spokesperson Jen Psaki told reporters Monday that Mosul Dam “has been in the sights of [ISIS] since its offensive began in June to further threaten and terrorize the Iraqi people.”

In addition to flooding concerns, the dam is also a “key source” of power and water for the surrounding area – making it a vital piece of infrastructure either way, another State Department spokesperson told ABC News Wednesday. An American intelligence source agreed and said that ISIS's potential control over and exploitation of power and water is a focus of U.S. intelligence community.

The Iraqi official involved in the dam’s operations refused to respond to the dire hypothetical of ISIS control Wednesday, but a U.S. government official long-familiar with the dam said it’s an unsettling idea made more so by a litany of unanswered questions. ISIS may not want the dam to fail, considering it controls territory that would be flooded and could leverage their control over the water and power source, but the U.S. official said it would still be up to the jihadist group to keep the grouting going.

“If ISIS does indeed have or gain control of the dam, will they listen to the dam engineers who have been working there for decades and who understand the need for constant grouting? … And then this is the biggie: If they can’t or don’t want to grout, how long will the dam last?... And if it fails, will it be a catastrophic all-at-once failure or more of a slowly building uncontrolled release?” the official told ABC News. “The short answer is no one knows. This is all guesswork anyway.”

The official said that he is not aware of official U.S. calculations about how long the dam would last without grouting but says he understands it to be “on the order of weeks, not months.” The geotechnical expert agreed that "weeks" was a skeptical, but entirely possible estimation.

“The potential for a disaster can’t be ruled out and should be of great concern to all parties involved,” the U.S. official said.

The U.S. State Department told ABC News late Wednesday the department is “monitoring the situation closely.” Officials at the Pentagon did not immediately respond to questions about whether any contingency plans are in place in case ISIS does take over the dam.

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