A sonar image of a new methane plume discovered off the US east coast
Researchers say they have found more than 500 bubbling methane vents on the seafloor off the US east coast.
The unexpected discovery indicates there are large volumes of the gas contained in a type of sludgy ice called methane hydrate.
There are concerns that these new seeps could be making a hitherto unnoticed contribution to global warming.
The scientists say there could be about 30,000 of these hidden methane vents worldwide.
Previous surveys along the Atlantic seaboard have shown only three seep areas beyond the edge of the US continental shelf.
Deep seep
The team behind the new findings studied what is termed the continental margin, the region of the ocean floor that stands between the coast and the deep ocean.
In an area between North Carolina and Massachusetts, they have now found at least 570 seeps at varying depths between 50m and 1,700m.
Their findings came as a bit of a surprise.
"It is the first time we have seen this level of seepage outside the Arctic that is not associated with features like oil or gas reservoirs or active tectonic margins," said Prof Adam Skarke from Mississippi State University, who led the study.
The scientists have observed streams of bubbles but they have not yet sampled the gas within them.
However, they believe there is an abundance of circumstantial evidence pointing to methane.
Most of the seeping vents were located around 500m down, which is just the right temperature and pressure to create a sludgy confection of ice and gas called methane hydrate, or clathrate.
The scientists say that the warming of ocean temperatures might be causing these hydrates to send bubbles of gas drifting through the water column.
They do not appear to be reaching the surface.
"The methane is dissolving into the ocean at depths of hundreds of metres and being oxidised to CO2," said Prof Skarke.
Methane hydrates recovered in the Gulf of Mexico by the US Geological Survey
"But it is important to say we simply don't have any evidence in this paper to suggest that any carbon coming from these seeps is entering the atmosphere."
This research, though, does highlight the scale of methane that is under the waters.
Estimates suggest that these undersea sediments are one of the largest reservoirs on Earth, and contains around 10 times more carbon than the atmosphere.
Carbon budget revisions
Prof Skarke and his colleagues estimate that worldwide, there may be around 30,000 of the type of seeps they have discovered.
They acknowledge that this is a rough calculation but they believe that it could be significant.
While the vents may not be posing an immediate global warming threat, the sheer number means that our calculations on the potential sources of greenhouse gases may need revising.
The scientists also found abundant life around many of these seeps, but not perhaps as we know it.
The creatures they describe are termed chemosynthetic, meaning they derive energy from chemical reactions and not from the Sun as do photosynthetic organisms.
Others who have collaborated on the search for seeps say these discoveries are important.
"These are significant geochemically, as they and our research teams found perhaps one of the largest seeps yet discovered with very active methane bubbling and large amounts of frozen hydrates," said Prof Steve Ross, from the University of North Carolina, Wilmington.
"These seeps are also significant biologically, as we have found unique chemosynthetic communities, huge range extensions and increased biodiversity."
As to the energy potential of these new seeping sources, Prof Skarke is fairly pessimistic.
"There is no evidence to say that these clathrates are related to conventional gas reservoirs, so there is no evidence to say they are a recoverable resource."
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