The U.S. Army has begun the process of destroying the nation's last supplies of chemical weapons which are contained in almost 800,000 artillery shells. The terrifying arsenal is contained at the Pueblo Chemical Depot in southern Colorado. The first batch of the mustard agent was destroyed after scientists used explosives to open up a sealed container and flooded it with a second chemical to neutralise the liquid. Scroll down for video . Technicians will take four years to eliminate almost 800,000 artillery shells containing mustard agent . The chemical agent, commonly known as mustard gas, is actually a liquid which smells like the condiment . It is estimated that there are 2,600 tonnes of the chemical agent contained within 780,000 shells at the facility. Bruce Huenefeld, who is managing the destruction process said: 'Everybody's really excited, but we're being cautious, making sure all the procedures are followed exactly.' Mustard agent can maim or kill by damaging skin, the eyes and airways. It's being destroyed under a 1997 international treaty banning all chemical weapons. It will take four years to destroy the Pueblo stockpile. Another 523 tons of mustard and deadly nerve agents are stored at Blue Grass Army Depot in Kentucky. Blue Grass isn't expected to start destroying its weapons until 2016 or 2017, finishing in 2023. The destruction process is safe, officials said. Most of Pueblo's stockpile will be dismantled and neutralised in a highly automated $4.5 billion plant built at the depot. The Pueblo Chemical Depot  in southern Colorado has a method of destroying damaged chemical shells . The United States said it never used mustard agent in any conflict and agreed to destroy their stockpile . About 1,400 damaged shells and a dozen metal bottles of mustard agent are considered unsuitable for that plant. They'll be opened with explosives and neutralised in the sealed chamber, which sits inside an airtight structure near the larger automated plant. The metal bottles contain mustard that was extracted from the shells for testing. A single bottle was the first container to be opened and neutralised Wednesday. Crews were waiting for the neutralisation to finish before draining the chamber, rinsing it and then removing the remains of the bottle. Once all the bottles are destroyed, crews will start work on the damaged shells, depot spokesman Thomas Schultz said. The automated plant isn't expected to begin work until December or January. Design and construction have taken years, and final testing and training are underway. 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 never used them in war. Nearly 90 per cent of its original stockpile has already been destroyed, mostly by incineration. The depots in Colorado and Kentucky are using chemical neutralisation because residents and officials expressed concerns about the vapor from incineration.
The United States agreed to destroy its mustard chemical agent in 1997 . The chemical agent is commonly called mustard gas despite being a liquid . Almost 800,000 artillery shells are being destroyed over the next four years . The United States stockpiled almost 3,000 tonnes of the liquid .