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Mosul Dam was built in the mid-1980s on soft gypsum rock, which is constantly eroding

U.S. experts warn the dam is at risk of "catastrophic failure." If it breaks, 1.5 million people could be at risk

Workers pump about 2 tons of concrete into boreholes in the dam's foundations every day to shore it up

Mosul Dam, Iraq CNN  — 

Only one of the two bottom outlet gates is functioning, angrily spitting out water in a foamy white churn as birds fly above the spray.

This is the Mosul Dam, considered by some experts among the most dangerous of its kind in the world.

Map Mosul Dam

Completed in the mid-1980s, the dam was considered a necessity at the time: Turkey was building its own barrages further upstream, and the government of then-President Saddam Hussein feared they could cause a water shortage.

Almost 2 miles (3.2 kilometers) long and 371 feet (113 meters) tall, it is Iraq’s largest hydroelectric dam, producing power for thousands of homes.

There’s just one problem: It is built on soft gypsum rock, which is constantly eroding.

Underground repairs to mitigate the damage and keep its foundations intact are going on almost constantly.

An employee works at strengthening the Mosul Dam on Iraq's Tigris River.

Daily repairs needed

A long, cool passageway leads into the underbelly of the dam, meeting up with another wider, darker tunnel; the clanking of machinery reverberates against the walls, and the air is thick with gasoline fumes.

The machines are drilling boreholes that will be filled with concrete – a process called grouting – which needs to happen daily.

On average, about 2 tons of concrete are pumped into the boreholes that line this entire 1.4 mile (2.2 kilometer) stretch of the dam every single day to solidify its foundations.

Two tons of concrete are pumped into boreholes in the foundations of Mosul Dam every day.