The Pueblo Chemical Depot has held about 780,000 shells of mustard gas, the nation's largest remaining stockpile of chemical weapons. But Wednesday, the army began the disposal process by moving the weapons to the disposal site, reports the Pueblo Chieftain.

The first container of mustard agent was moved into an airtight containment building at Pueblo Chemical Depot in Colorado on Wednesday, and crews expected to open it and neutralize it later in the day.

Pueblo is one of the last two sites in the country with chemical weapons stockpile, says CNN. The U.S. signed a treaty to eliminate its chemical weapon stockpile by 2007, but the deadline was extended to 2012, noted the Gazettte

It will take four years to destroy the Pueblo stockpile. About 1,400 damaged shells will be processed first. The Gazette reported in January that the $725 million robotic destruction plant will operate around the clock to neutralize the weapons.