The view from a helicopter as it flies above the dam in Mosul, Iraq, is seen in this Sunday, Sept. 25, 2005 file photo.

Iraq’s Mosul Dam has long been branded the world’s most dangerous dam, at risk of collapsing and sending water crashing over millions of people. That prospect is even greater than was previously believed after ISIS captured the dam briefly in 2014, according to a new report by U.S. Army engineers.

Efforts to find a permanent solution for the dam, Iraq’s biggest, are held up by political wrangling and the price tag of more than $2 billion, leaving the country dependent on stopgap measures that some experts fear will no longer be enough. Iraq’s Water Resources Ministry says there is no imminent danger of collapse, despite the U.S. warnings.

The dam’s core problem is that it was shoddily built on unstable ground: The earth underneath it is constantly being eroded by water. From the day it was inaugurated in 1985, maintenance crews have had to continuously pour cement under its foundation.

Without that constant injection — known as “grouting” — the 113-meter-high dam would soon collapse into a hole in the ground, causing an unprecedented disaster. The 30-mile long lake behind it would explode down the Tigris River valley with hundreds of millions of cubic meters of water, ramming into Iraq’s second-largest city, Mosul, currently home to more than 700,000 people about 40 miles downriver. It would then flood all the way down to Baghdad, some 340 miles south.

U.S. officials have estimated more than a half million people could be killed. Millions more would be driven from their homes.

“Mosul Dam is at a significantly higher risk of failure than originally understood and is at a higher risk of failure today than it was a year ago,” said the report, which emerged in a parliament report made public on Monday.

The report gave no estimated timeline for a potential collapse, but U.S. officials and engineers warn it could happen at any time. Grouting can extend the dam’s lifespan, but any disruption could quickly turn disastrous.

“The likelihood of the dam collapsing is something we are trying to determine right now,” U.S. Army Lt. Gen. Sean MacFarland told reporters in Baghdad in January. “All we know is when it goes, it’s going to go fast and that’s bad.”

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