Hexachlorocyclopentadiene (HCCPD), also known as C-56, Graphlox and HRS 1655, is a lemon-yellow liquid that easily changes into a blue haze vapour. It is an organochlorine compound that is a precursor to several pesticides, flame retardants and dyes. This colourless liquid is an inexpensive reactive diene. Many of its derivatives proved to be highly controversial, as studies showed them to be persistent organic pollutants. Collectively, the pesticides derived from hexachlorocyclopentadiene are called the cyclodienes. An estimated 270,000 tons were produced until 1976, and some are still manufactured by Velsicol Chemical Corporation in the US and by Jiangsu Anpon Electrochemicals Co. in China. (source)