The world's largest frozen peat bog is melting, which could speed the rate of global warming, New Scientist reports.
The whole western Siberian sub-Arctic region has started to thaw
The huge expanse of western Siberia is thawing for the first time since its formation, 11,000 years ago.
The area, which is the size of France and Germany combined, could release billions of tonnes of greenhouse gases into the atmosphere.
This could potentially act as a tipping point, causing global warming to snowball, scientists fear.
The situation is an "ecological landslide that is probably irreversible and is undoubtedly connected to climatic warming," researcher Sergei Kirpotin, of Tomsk State University, Russia, told New Scientist magazine.
The whole western Siberian sub-Arctic region has started to thaw, he added, and this "has all happened in the last three or four years".
Western Siberia has warmed faster than almost anywhere on the planet, with average temperatures increasing by about 3C in the last 40 years.
The warming is believed to be due to a combination of man-made climate change, a cyclical atmospheric phenomenon known as the Arctic oscillation and feedbacks caused by melting ice.
The 11,000-year-old bogs contain billions of tonnes of methane, most of which has been trapped in permafrost and deeper ice-like structures called clathrates.
But if the bogs melt, there is a big risk their hefty methane load could be dumped into the atmosphere, accelerating global warming.
Scientists have reacted with alarm at the finding, warning that future global temperature predictions may have to be revised.
"When you start messing around with these natural systems, you can end up in situations where it's unstoppable," David Viner, of the University of East Anglia, UK, told the Guardian newspaper. "There are no brakes you can apply.
"This is a big deal because you can't put the permafrost back once it's gone. The causal effect is human activity and it will ramp up temperatures even more than our emissions are doing."
The intergovernmental panel on climate change speculated in 2001 that global temperatures would rise between 1.4C and 5.8C between 1990 and 2100.
However these estimates only considered global warming sparked by known greenhouse gas emissions.
"These positive feedbacks with landmasses weren't known about then," Dr Viner said. "They had no idea how much they would add to global warming."