Early Hanford scientists understood that these chemical and radioactive wastes could be hazardous to people or the environment, and decided to build a series of massive underground storage tanks ranging in capacity from 55,000 gallons to more than 1,000,000 gallons to hold the wastes. Scientists believed that the tanks would only be used temporarily until a permanent place to dispose of the waste was identified. Still, they required that the early tanks be constructed with robust materials consisting of a carbon steel shell surrounded by reinforced concrete. This proved to be a fortunate decision since many of the tanks remain in use to this day. No new waste from plutonium production has been added to the tanks in many years, but some of the waste that was originally put in them is still there.