"Still burns really well," Ryabinin says. "It's very likely that these puddles are stretching all over the river and will be polluting it for a very long time."

We were a few kilometers from the Siberian city of Norilsk, where six weeks ago a huge fuel tank at a power plant ruptured, spilling thousands of tons of diesel into the river.

The owner of the plant, the Nornickel metals giant, says the spill was quickly contained, and the damage limited. Ryabinin has sacrificed his job and his family's future in Norilsk in an attempt to lift the lid on what environmentalists have called the worst ecological catastrophe in the polar Arctic.

It was 2 a.m. in the Arctic summer. A half-light illuminated the fast-moving river as it flowed through the endless tundra towards the Arctic ocean. A rainbow film of oil covered the surface; a pool of diesel squelched beneath our feet.

Ryabinin brought us there by foot along railroad tracks. Ever since the spill, the areas surrounding the site have been guarded by security personnel, making them difficult to access.

He is a rare creature in today's Russia -- a whistleblower who quit his job with the state environmental agency Rosprirodnadzor and went public about the extent of the disaster.

Ryabinin says he was first alerted to the scale of the crisis on May 29 by photographs posted on Instagram. He was immediately alarmed: the Daldykan and another river polluted by the spill flow into Lake Pyasino. From there, the contamination could spread all the way to the Arctic Ocean.

Just a few hours later he was at the river, taking photographs that would soon provoke a public outcry. He and his boss tried to get in to the Nornickel plant, but he says they were refused entry by police.

More than 20,000 tons of diesel poured into the rivers from the storage tank, according to Nornickel.

Foaming red sludge mixed with the water and sucked life from the rivers and their banks.

"It looked horrible when we got there and it wasn't even the worst of it as a couple of hours had passed," Ryabinin says. "You could smell the diesel half a kilometer away... my boss was even afraid to smoke there in case it blew up."

What he saw was very different from what officials and media were later reporting: that the spill had swiftly been brought under control. Russian state TV ran reports showing aerial pictures of oil-spill booms guarding the crimson layer of diesel.

"It was such an obvious, childish lie, I couldn't wrap my head around it," Ryabinin told CNN.

"Obviously I thought we must at least investigate the lake but my [agency] had a different view, which corresponded with the one of the [Nornickel] plant — that the spill did not spread further that the river."

Ryabinin says the last straw for him was when Rosprirodnadzor told him to stop looking into the disaster after he had found a helicopter to fly to the lake. At that point, on June 7, he went public, recording a 45-minute account of what he'd found -- concluding th

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