In the United States, there are two remaining stockpiles of the liquid that makes mustard gas.

One's in Kentucky. And the largest reserve sits right here in Colorado.

The United States Army plans to start operating a $4.5 billion plant next week that will destroy the nation's largest remaining stockpile of mustard agent, complying with an international treaty that bans chemical weapons, officials said on Wednesday.

The largely automated plant at the military's Pueblo Chemical Depot in southern Colorado will begin destroying about 780,000 chemical-filled artillery shells soon after this weekend, said Greg Mohrman, site manager for the plant. He declined to be specific, citing security concerns and possible last-minute delays.

The shells have been sitting underground in what are called igloos since the 1940s or 50s, according to Rick Holmes, the project manager for the Bechtel Corp.-led team that designed and built the facility where the chemicals will be destroyed.

"We've practiced a lot," Mohrman told The Associated Press news agency. "Next week it gets real."

By practice, he means using fake chemicals to rehearse the routine, and any possible scenario, in the event that something goes wrong, Holmes said.

But the people handling this operation aren't working alone. Robots will dismantle the shells, and the plant will use water and bacteria to neutralize the mustard agent, which can maim or kill by damaging skin, the eyes and airways. At full capacity, the facility can destroy an average of 500 shells a day operating around the clock.

It's expected to finish in mid-2020.

The plant will start slowly at first and likely won't reach full capacity until early next year, said Holmes. He says the team is ready, and so is Colorado.

"Nervousness? No. This is what they've trained to go do. This is their job. Getting everybody aligned is really hard. So to get to this point where people are ready, it's a pretty significant accomplishment by the entirety of the team. And it's time for Colorado. It's time to destroy these munitions, make Colorado safer, and then move on to what's next."

The depot has already destroyed 560 shells and bottles of mustard agent that were leaking or had other problems that made them unsuitable for the plant.

Those containers were placed in a sealed chamber, torn open with explosive charges and neutralized with chemicals. That system can only destroy four to six shells a day.

Irene Kornelly, chairwoman of a citizens advisory commission that Congress established as a liaison between the public and the plant operators, said her group had no remaining safety concerns.

The shells stored at the Pueblo depot contain a combined 2,600 tons of the chemical. Some of the end product after it's destroyed can be recycled and used again. Holmes says any vapor that's released into the environment is likely cleaner than the rest of the air.

Once the project is complete, the plant will be torn down.

The Army stores an additional 523 tons of mustard and deadly nerve agents at Blue Grass Army Depot in Kentucky. Blue Grass is expected to start destroying its weapons next year, finishing in 2023.

Mustard agent is a thick liquid, not a gas as commonly believed. It has no color and almost no odor, but it got its name because impurities made early versions smell like mustard.

The U.S. acquired 30,600 tons of mustard and nerve agents, but it says it never used them in war. Nearly 90 percent of its original stockpile has already been destroyed, mostly by incineration.

A 1925 treaty barred the use of chemical weapons after debilitating gas attacks in World War I, and the 1997 Chemical Weapons Convention called for eradicating them.

But international inspectors say Syria and the Islamic State of Iraq and the Levant group used them in 2014 and 2015. The United Nations Security Council met in closed session on Tuesday to consider whether to sanction Syria.