This is the Mosul Dam, considered by some experts among the most dangerous of its kind in the world.
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.
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.
But years of neglect brought on by a lack of funding, corruption and bureaucracy have plagued efforts to refurbish the dam, and Iraq's security problems have kept much-needed international companies -- and their expertise and advanced technology -- away.
In a recent security message, the U.S. government said if the dam collapses, "Some models estimate that Mosul could be inundated by as much as 70 feet (21 meters) of water within hours of the breach. Downriver cities such as Tikrit, Samarra, and Baghdad could be inundated with smaller, but still significant levels of flooding within 24-72 hours of the breach."
The lives of at least 1.5 million people could be at risk, according to U.S. estimates.
Under ISIS control
In August 2014, ISIS seized control of the dam, holding it for around 10 days. More than 1,500 staff and their families fled as the clashes intensified.
When the first team returned to assess the damage, its driver was killed as he entered a booby-trapped bathroom. The team withdrew from the facility until Peshmerga explosives teams could be brought in to clear the site.
In all, the ISIS occupation, looting, damaged machinery and a lack of staff meant the grouting process was halted for 45 days.
Riyad al-Naemi, the dam's manager, acknowledged things are far from ideal, but said he is confident his team has repaired the weaknesses in the barrier's foundations.
But the United States has its doubts about the current quality and quantity of the grouting, and fears that chasms opening up beneath the dam could lead to its collapse. Experts also warn that the much-needed repairs are putting added strain on the fabric of the dam.
In 2015 the United States installed sensors -- an early warning system -- to monitor the structure's stability.
Naemi said measures have been taken and said the damage the possible collapse of the dam would cause is not as severe as has been reported.
"The level of the lake was reduced from 330 meters above sea level to 319 meters," he said. "This is considered to be a safe level."
"If the dam were to collapse when the water level is at 330 meters above sea level, yes the city of Mosul would be entirely flooded, but with the current levels we have there will be minimum impact to Mosul."
He said, though that "the villages right along the bank of the Tigris, yes, they would be impacted."
It's a delicate balance, especially as this winter's snows melt, causing the lake's levels to rise.
And with only one of the dam's bottom gates working, that brings more problems. It is generating uneven pressure on the foundations, and further downstream the currents it creates are leading to erosion along the banks of the Tigris.
An Iraqi team tried to repair the jammed outlet after it broke in 2013, but further problems led to flooding in the control room, and two employees who were stuck there drowned.
Finally, sensing the urgency of the situation, the Iraqi government got its country's bureaucratic gears in motion and issued a contract worth around €273 million to an Italian company.
The Italians will bring new equipment, and they are also tasked with carrying out repairs and staff training. But all this has yet to begin.
In the meantime -- in a nation where nothing is ever entirely predictable -- it is always best to plan for the worst.