The world's biggest Nickel producer has admitted a spillage at one of its plants was responsible for a river in the Russian Arctic turning blood-red.
However, it says there is no danger to people or wildlife.
The company had flatly denied it was responsible when Images of the red river near Norilsk emerged last week.
A Greenpeace Russia activist says it is too early to judge the impact.
"You can't just say that it's no big deal," Alexei Kiselyov told the AFP news agency.
He said Norilsk Nickel controlled access to the entire Taymyr Peninsula, where the incident happened, hampering investigators looking into pollution from its plants.
Environment Ministry officials had suggested last week that a leak of chemical pollutants from a pipe at the industrial site could have discoloured the river.
Norilsk Nickel denied any such claim, however, even posting pictures allegedly showing the river with a "natural tone" on 7 September.
Indigenous groups have accused the company of lax safety standards.