During World War II, the U.S. military assembled a special group of troops trained to use chemical weapons.

"The big day is still to come," says the narrator of a U.S. War Department training film. "It will be when Hitler, with his back to the wall, frantically uses gas as a last resort."

It was the worst kind of warfare imaginable in the years before the atomic bomb.

A network of labs and arsenals supported the production of America's chemical weapons. The "restricted" film that was kept from the public noted one site on the outskirts of Denver: Rocky Mountain Chemical Warfare Arsenal. Among the chemical weapons made there were hundreds of thousands of mustard gas weapons.

Known for its garlic-y smell, the gas could kill, if not incapacitate entire platoons, leaving them blistered, blinded and struggling to breathe.

After the nightmare battle with the Nazis never happened, the weapons were put on rail cars in 1953 and 1954 and sent 100 miles south to an Army depot in Pueblo, according to the Army. They sat there for decades but now, after years of planning, testing, and ramping up, are being dismantled at a steady pace.

HITTING A MILESTONE

The heavily-guarded industrial plant located on the Army's Pueblo Chemical Depot is doing the work. The Pueblo Chemical Agent-Destruction Pilot Plant is one of just two sites in the United States still contending with such weapons. Under the international Chemical Weapons Convention, they all must be destroyed.

The date for destroying all 780,000 weapons stored at the Pueblo depot is December of 2022. That's year ahead of the deadline set by Congress, said plant site project manager Walton Levi.

Credit U.S. Army / U.S. Army Deadly mustard agent was manufactured in this building pictured in 1943 on Rocky Mountain Arsenal. Chemical weapons were later sent to an Army depot in Pueblo.

"We want to make sure we target early as a contingency," Levi said.

That's because there can be delays. On May 15, operations were halted for several weeks when leaks were discovered on two large chemical storage tanks.

"Between the two tanks it was a total of eight ounces," Levi said. "We are very vigilant. Safety is everything we do."

Operations were restarted on June 12, after a third tank was deemed safe.

Since then, the plant has gone on to report a milestone. About 150,000 of the largest shells — 155-millimeter — have been destroyed. Each looks like a giant brass bullet about the size of a fire extinguisher, weighs nearly 100 pounds and holds a little more than a gallon of mustard agent.

INGA AND IGOR

Decades ago, the Army disposed of chemical weapons by burying or open burning them — even dumping them into the sea. When those methods fell under scrutiny, the Army turned to incineration, including at Rocky Mountain Arsenal in the 1970s to dispose of 3,000 tons of mustard agent and 4,000 tons of sarin. The arsenal is now a wildlife refuge.

Credit Pueblo Chemical Agent-Destruction Pilot Plant / Pueblo Chemical Agent-Destruction Pilot Plant Igor is the name of one of two robots at the Pueblo Chemical Agent-Destruction Pilot Plant's training center. The other is Inga. The robots are named after characters in the 1970s classic film, 'Young Frankenstein.' This one bears the image of actor Marty Feldman.

When incineration was proposed for the mustard shells and mortars kept at the Pueblo depot, there was an uproar, said citizen watchdog Irene Kornelly.

"There was a great deal of concern," she said, particularly among farmers and ranchers. "A lot of the agriculture community around Pueblo is very high-grade organic crops."

Kornelly is a locally-appointed member of the Colorado Chemical Demilitarization Citizens' Advisory Commission. It's a group that monitors every aspect of the destruction process, giving voice to the community. She said what's happening at the plant in Pueblo is an alternative to incineration that took years of discussion and more than a decade of planning — from the conception of the plant starting in 2004, to construction starting in 2008, to subsequent testing.

Robots assist workers at the plant, which started destroying the first of the depot's weapons in late 2016.

A building near the depot is a replica of operations, but without the hazard of mustard gas. There, Inga and Igor, robots named for characters in the classic 1970s Mel Brooks film "Young Frankenstein," are part of the training.

The robots cut open the tops of the shells and remove the bursters that would make them explode. Then comes the tricky part. Some shells are corroded inside and, as a byproduct, hydrogen gas can mingle with the mustard agent.

Credit Pueblo Chemical Agent-Destruction Pilot Plant / Pueblo Chemical Agent-Destruction Pilot Plant Pueblo Chemical Agent-Destruction Pilot Plant workers train in protective suits.

"It's in the liquid, just like champagne," said Tom Bailey, plant support specialist. "What happens when you open a champagne bottle? All those bubbles break and when they do, it expands. That liquid becomes foam and it expands to where it wants."

That would be a disaster for the workers. At the ready are what look like space suits equipped with air and pulse monitors.

"There's chemical agent in there," Bailey said. "There's decontamination solution in there, a vapor hazard, a liquid hazard. It'll kill you."

'PLAIN OL' SALT'

Igor delicately maneuvers a roughly 100-pound shell upside down at a kind of sink. If the gallon of liquid inside were to start bubbling, this method should handle it, Bailey said. A machine then sprays high-pressure, hot water into the shell, coaxing out more what's inside. Igor then holds it for 30 seconds to allow any liquid left to drain.

The empty shells goes one way — to an oven where they're super heated, burning off any remaining mustard. The steel is recycled.

As for the liquid, it flows down a drain through titanium pipes in a closed system.

Levi, the site project manager, said what happens next is "just basic wastewater treatment 101."

The mingling with water began the process of breaking down the mustard agent, which eventually goes to a reactor to be neutralized. It becomes what's called thiodiglycol and the same kind of microbes that consume human waste in sewage systems also eat the stuff.

Credit Pueblo Chemical Agent-Destruction Pilot Plant / Pueblo Chemical Agent-Destruction Pilot Plant These 'salt cakes' are what is left after shells containing mustard agent are processed at the pilot plant in Pueblo. The are considered hazardous waste and sent to a landfill.

"It's Darwin at its best," Levi said. "So those microbes that really like the thiodiglycol and can digest it best, they multiply and take over."

What's left is "mostly water and salt … plain old salt," Levi said. "It's kind of a slurry solution coming out. It's kind of a wet salt."

Water is pressed out of that salt and reused at the plant. That leaves salt cakes, which are contaminated with heavy metals.

"So you'll see these salt cakes go off to a hazardous waste landfill," he said.

RAMPING UP

The plant is destroying about 24 weapons per hour, a rate agreed to with state health officials, Levi said. The plant could go faster, perhaps as fast as "a total of 64 an hour."

Yet it is not yet clear how fast the plant should go. The plant is providing environmental information the state to find an optimal rate within safe emission limits.

"The Colorado Department of Public Health and Environment will set the limit as to how many weapons per day they are allowed to put through," said Kornelly with the citizens group. "They have to go through a pretty massive process called a multipathway health risk assessment of everything that comes out of the plant. Does it still meet air quality standards? Does it still meet all the standards that we're looking for?"

Levi is confident the plant will meet environmental and public health standards and that the last of the depot's chemical weapons can be destroyed by the end of 2022.

The state is expected to issue draft permit language for the plant's destruction process in the coming months — as well as for another type of destruction method that would handle mustard mortar rounds in static detonation chambers. When those permits are issued, it will trigger a public comment period.

As long as the community's safety is assured, Kornelly expects the plant to get a green light from the state. When the last weapons are destroyed at Pueblo's Army depot, and another depot in Kentucky, the United States will then declare itself free of its chemical weapons, which were manufactured for a battle to end all battles that never happened.