Russian environmental inspectors are trying to establish a river near the Norilsk Nickel industrial complex in the Arctic has turned blood-red.
Stunning graphics of the discoloured Daldykan river have been posted extensively on Russian media.
The government daily Rossiiskaya Gazeta says a leaking slurry pipeline taking waste copper-nickel concentrate could be to blame.
Norilsk Nickel is the world’s biggest nickel and palladium producer.
Its vast furnaces were constructed in the Soviet age, in the Krasnoyarsk region of Siberia, on the Taimyr Peninsula.
The mining group has a production facility called Nadezhda by the Daldykan river. But company officials said they were unaware of any river pollution in the plant.
Billionaire oligarch Vladimir Potanin is president of Norilsk Nickel.