You can see them from shore along the Arctic coast or even in some northern lakes - seething domes of water churned up by gas escaping from deep below.
"The largest ones have the feeling of a hot tub," says Scott Dallimore, a scientist with Natural Resources Canada.
"They look like floating hot tubs out in the water. They're bubbling cauldrons of gas. They're quite spectacular. "
"They're pure methane."
And that's the worry.
Unimaginable quantities of methane - a greenhouse gas 20 to 25 times more potent than carbon dioxide - are stored underground in the Arctic. Some of it is leaking out.
The consequence of all that seeping methane has become one of the biggest questions in climate science.
But one thing is certain: The fact it hasn't been factored into previous global warming predictions means forecasts even as recent as the 2007 report from the Intergovernmental Panel of Climate Change are too conservative.
"[Methane]was not considered in any of the predictions at all," says Andrew Weaver, a Canadian researcher and one of the IPCC authors.
Methane, a carbon compound, is stored in organic material frozen into permafrost and in vast underwater deposits called hydrates - a strange, slushy blend of methane and water.
Estimates of how much is out there are vague. There could be anywhere between 500 to 10,000 gigatonnes of carbon in the hydrates and another 7.5 to 400 gigatonnes in the permafrost.
But some say there's enough carbon in underground methane - including large deposits under tundra lakes in the Mackenzie Delta and along Canada's Beaufort Sea coast - to equal the carbon from all the Earth's remaining deposits of oil, coal and natural gas combined. Last week, a World Wildlife Fund report called methane the globe's single biggest climate threat.
Methane escapes from underground into the atmosphere as the earth around it warms up. Some of that warming is from recent climate change but some of the deepest warming is in response to events that happened up to 12,000 years ago.
"Is our recent warming affecting it more or less?" asks Mr. Dallimore. "That's a very reasonable question to ask."
Scientists also don't know how much of the methane is coming from deep deposits and how much is from relatively shallow beds.
"There's a building international awareness that this is a gap in our knowledge that should be addressed," says Mr. Dallimore, who has recently returned from his research season in the Mackenzie Delta of the Northwest Territories. "The challenge is to quantify what portion of that large reservoir of methane is presently stable or not."
Some researchers suggest the methane seeps have been releasing gas for centuries, if not millennia, and are creating concern simply because they've been discovered. But others point to signs that methane releases may be increasing.
One team found that methane emissions from a northern Siberia deposit increased by 58 per cent between 1974 and 2000. As well, average global methane concentrations suddenly spiked in 2007 after remaining stable for most of a decade.
Samples of seawater from the Arctic Ocean have shown methane levels two to 10 times higher than previous years. Methane bubbles up to 30 centimetres wide have appeared trapped in the sea ice off Siberia.
Many suggest that methane could be a climate "tipping point." Seeping methane will add to global warming, which will lead to ever larger and increasingly catastrophic amounts of the gas in the atmosphere.
Mr. Weaver says sudden, large releases are very unlikely.
"The catastrophic effect is not there," he says.
But methane is going to be a factor in future climate change.
"We know that it's a positive feedback," says Mr. Weaver.
More and more scientists are beginning to study methane deposits, Dallimore says. New tools, such as devices that can measure methane seepage from the air, are sharpening knowledge of what's going on under the countless lakes of the tundra and vast sweeps of Arctic ocean.
"It's all connected and it's building up greenhouse gas concentrations by natural sources," says Mr. Dallimore.
"The question is how much?"Report Typo/Error