The giant industrial complex at Norilsk, in Russia's remote Artic region, is accused of being world's largest producer of acid rain.
Foreigners are usually forbidden from entering the secretive region.
Ore extracted from Norilsk mines produces a rich mix of metals including nickel, copper, palladium, platinum, gold and silver, but it also contains a large amount of sulphur.
Chief engineer Sergei Romanov shows a small piece of ore at one of the smelters.
The metal industry in Norilsk was built from scratch by prison labourers of Stalin's brutal Gulag.
Norilsk residents go about their lives as the smelters pour smoke on to the city.
Many of the trees near the smelters have been killed off by acid rain or other pollutants.