The exact reason of the leak is yet to be established, but a statement from Norilsk Nickel company, which operates the site suggests it could have been caused - worryingly - by collapsing permafrost.

President Vladimir Putin declared a state of emergency last Wednesday, several days after 21,000 metric tons of diesel leaked from a collapsed fuel tank outside the city of Norilsk.The pollution now risks running north into the Arctic Ocean.

While mining giant Nornickel has said the local ecosystem is "satisfactory," environmentalists paint a different picture.