Researchers also believe that systems like Lost City may be common on Earth, according to Deborah Kelley, University of Washington associate professor of oceanography and one the three people on the first manned dive to the field the day after it was discovered.

To speculate that life on this planet may have started in the relatively warm, alkaline waters rich with methane and hydrogen that result when seawater reacts with mantle rock.

Today the mantle in most places is capped by oceanic crust. But early in Earth's history mantle rocks may have been much more exposed to seawater, providing ample opportunity for conditions to support microbial life at the seafloor.

Kelley says Lost City is distinctive because it sits on mantle rock that's about 1.5 million years old and its hydrothermal venting -- in which water circulates into the seafloor, gaining heat and chemicals until there is enough heat for the fluids to rise buoyantly and vent back into the ocean -- is much different from previously explored vent fields.

It doesn't appear that volcanic activity drives hydrothermal venting at Lost City; fluids from there give no chemical evidence of having been in contact with magma chambers.

This is unlike any system found at the Earth's spreading centers where magma chambers are present. At speading centers very young seafloor is created -- often dramatically during volcanic eruptions -- and vented water can be as hot as 700 degrees F.

Instead, venting at Lost City occurs because of heat generated by chemical changes in the rocks: seawater permeates deeply into the fractured surface of the mantle rocks where it transforms the mineral olivine into a new mineral, serpentine, in a process called serpentinization.

The heat generated during serpentinization is not as great as that at volcanically active sites but it is enough to power hydrothermal circulation and produce vent fluids of 105 to 170 degrees F, Kelley says. The result is a field of dramatic vents not made of sulfide but of carbonate minerals, or limestone.

The most massive, at 18 stories, is the tallest vent structure ever seen anywhere. The vents support a community of microorganisms able to live off the fluids rich with methane and hydrogen, both byproducts of serpentinization.

The Lost City hydrothermal vent field was discovered Dec. 4, 2000, in the mid-Atlantic, at about 30 degrees north latitude, during a National Science Foundation-funded expedition that had not set out to seek a hydrothermal vent field. It is nine miles from the nearest volcanically active spreading center.

It is on the summit of the mountain known as Atlantis Massif, one reason for the name Lost City.

Mantle rock is usually many kilometers beneath the seafloor but at the Atlantis Massif, the Earth's forces thrust mantle rock up exposing it directly at the seafloor. Spreading and faulting stripped much of the mountain down to the underlying mantle rocks.

The extent of the hydrothermal field is unknown. In the limited time researchers were there they saw about 30 active and inactive carbonate chimneys. Tallest is the 180-foot vent scientists have named Poseidon. Previously studied vents mostly reach 80 feet or less with the tallest being a 135-foot vent on the seafloor off the coast of Washington (which toppled in recent years).

The new vents are nearly 100 percent carbonate, the same material as limestone in caves, and range in color from a beautiful clean white to cream or gray, in contrast to black smoker vents that are a darkly mottled mix of sulfide minerals.

It's easy to imagine there could be many more such systems, Kelley says. Within a mere 60-mile radius of the Atlantis Massif are three similar mountains subject to the same fracturing, the same intrusion of seawater and perhaps the same reactions with mantle material. And those four represent a tiny fraction of the potential sites along the 6,200 mile Mid-Atlantic Ridge, Indian ridges and the Arctic Ridge, also considered slow- and ultra-slow-spreading centers.

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