The underwater volcanoes are part of a larger structure of tar deposits in the area, and although the volcanoes themselves are not active, oil has been bubbling steadily out of nearby seeps in the underground rock for thousands of years.

Just sail some 10 miles offshore and the surface of the ocean has a oily sheen and smells, says Chris Reddy, a scientist at WHOI and co-author of a paper on the asphalt volcanoes that appeared in April's Nature Geoscience. Some 20,000 liters a day is released; about half the oil that enters the world's oceans comes from natural seeps like these.

The volcanoes are dormant now, but at one time may have been an important regional flux of methane, a potent greenhouse gas. Finding natural sources of methane like these are critical to understanding how methane is released into the atmosphere.

"These sites likely put methane in to the atmosphere when they were active, but the amounts from individual sites were likely small compared to the global total," said Dave Valentine, a geochemistry professor at UC Santa Barbara and co-author of the paper.

Valentine and his colleagues discovered the volcanoes on a diving expedition in the area. They were curious about some unusual sea floor topography they had noticed, and then sent an autonomous underwater vehicle to snap some photographs.

What was revealed were seven domed volcanoes in all, larger in area than a football field, the largest of which was taller than a six story building. How do you miss these so close to a heavily populated coastline? Well, they were deep enough that diving expeditions never reached those peaks.

The volcanoes are thought to be made entirely of asphalt rooted deep below the subsurface to their base. It's like a massive bumpy parking lot down there. Valentine says they're not a source of global warming today, although the surrounding seeps are adding a relatively small, ever-present source to the atmosphere.

"These sources contribute to the background methane that exists in the atmosphere, and are rather minor compared to the impact of human activities over the last two centuries, rice agriculture and dairy farms being examples of sources impacted by humans," he says.

But geochemists are often thinking in terms of scale.

"In a longer term view, changes in emission from the subsurface may have had significant impacts on climate, but it would take many such features as we have found to make a global impact."

Human volcano

The volcano in Iceland is a reminder of how ultimately precarious our situation is here on Earth. There's just no telling what the planet's systems have in store for us. We build entire civilizations on the assumption of permanence. But in moments the ground - or skies - can start shifting.

As Eyjafjallajokull continued spurting dark clouds across Europe causing human chaos, scientists simply could not say for sure when the end of it was near.

No known end. And of course the scariest part is that this kind of thing isn't all that unusual in Earth's past. Although Eyjafjallajokull is happening in an inopportune spot, it's not particularly large either. Now's the time to bone up on past volcanic occurrences.

We've all heard of the 1991 eruption of Mount Pinatubo in the Philippines - the largest eruption in living memory - which caused average global temperatures to drop by almost a degree Fahrenheit as a haze of sulfuric acid droplets prevented sunlight from reaching the Earth. Some 800 people died from houses that collapsed under accumulated wet hot ash, and of course there was untold damage to nearby communities, forests, and agriculture. How fast things change: these days, the volcano is a tourist destination.

Well, step aside Mount Pinatubo. There are no images for this next one. The Mount Toba "super-eruption" about 75,000 years ago in Sumatra has been called the "eruption that brought humanity to its knees." It brought on a six year "volcanic winter" and then a thousand year snowball Earth. Scientists think a bottleneck in the human genetic code stemming from around that time could have been a massive die-off of humans, then hunter gatherers, to a population of only 5,000 individuals (see, we are all related). These days Toba's crater has become a peaceful lake where buffaloes graze and the ethnic Batak people work their rice patties.

So what's all this have to do with global warming? Well, volcanic eruptions temporarily (or not so temporarily) lead to global cooling, though, of course, also cause quite a bit of destruction so we don't exactly want them to pop off. But in the long term, they are actually adding quite a bit of CO2 to the atmosphere. Think of them as balancing out the Great Carbon Cycle.

But as Virginia Polytechnic Institute geologist Dewey McLean points out:

Today, our burning of the fossil fuels coal, oil, and gas is like a human volcano that is releasing vast amounts of carbon dioxide into the atmosphere.

We're a human volcano. He's kinda right.

Despite all the carbon it spewed into the air, Eyjafjallajokull has, in an odd way, actually helped alleviate climate change.

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