Methane chasers: Hunting a climate-changing gas seeping from Earth's seafloor

Methane is a potent greenhouse gas that can pack more than 25 times the global warming punch of carbon dioxide, and atmospheric methane emissions have been growing significantly since 2007. So it’s vital that humanity knows how and where methane emissions are coming from, including the world’s oceans.
Scientists first raised the alarm over methane releases from shallow waters in the Arctic Ocean between 2008 and 2010. But recently, they were surprised to discover new releases in shallow waters off Antarctica. Researchers continue spotting additional seafloor seeps there and elsewhere, as methane bubbles escape seafloor sediments.
In shallow waters, methane bubbles that break the ocean’s surface add greenhouse gases to the atmosphere, but to learn how much climate risk these bubbles pose, scientists first have to find them. The hunt for methane bubbles requires everything from underwater microphones and sonar maps to scuba divers and submersibles.
Methane seeps are more than a potential climate change threat. They also form the basis of unique chemosynthetic ecosystems that influence the deep sea and may hold clues about the origin of life. Finding and studying those seeps present fascinating challenges, requiring ingenuity and creative thinking by researchers.

They’ve been called “bubble chasers,” and “seep seekers,” though they sometimes call themselves “flare hunters.” They’re a small group of scientific specialists searching the world’s oceans for tiny streams of methane gas-filled globules rising from seafloor sediments.

On expeditions ranging from the Arctic to Antarctica, carried out in shallow waters to thousands of meters below the sea’s surface, their studies reveal how these tiny globules can potentially add to global warming while also creating unique ecosystems.

But even when deploying advanced modern technology, finding these cold-ocean methane seeps isn’t easy. And it may be even harder to determine exactly how seafloor methane releases could factor into the future of humanity and the planet.

Map showing the known global occurrences of methane-derived carbonates used to compile a study of seafloor methane seepage across the last 150 million years. Image courtesy of Oppo et al. (2020).

Bubbles flowing from a methane seep at El Quisco, off the coast of Chile. Researchers there found the seeps using a combination of sonar-based bubble mapping, bathymetric mapping, tracking in situ methane concentration measurements and visual surveys with the ROV SuBastian. — Bubbles flowing from a methane seep at El Quisco, off the coast of Chile. Researchers found the seeps using sonar-based bubble mapping, bathymetric mapping, tracking in situ methane concentration measurements, and visual surveys with the ROV SuBastian. Image by ROV SuBastian/Schmidt Ocean Institute (CC BY-NC-SA).

Hunting telltale bubbles

“These seeps are fascinating and extreme environments,” said Claudio Argentino, a sediment biogeochemist at UiT, The Arctic University of Norway, whose fieldwork started at ancient methane seep sites in Italy’s Apennine Mountains in 2015, during his doctoral studies, and now takes him to the Arctic Ocean.

“We want to know how much gas is escaping the seafloor sediment and getting into the atmosphere. It’s a simple question but very challenging to answer because you cannot just use satellite data. You really have to go there.”

Described as “ubiquitous but unique,” tens of thousands of methane seeps have been found along continental shelves around the world. The methane released is mostly generated by the microbial decomposition of organic matter that settles in seafloor sediment. It bubbles up when the Earth’s crust shifts or, in polar regions, when sea ice retreats and releases pressure on the seafloor. “It’s like popping a cork from a champagne bottle,” Argentino said.

Left image: Bubble shapes in water vary, being almost spherical up to a millimeter in size (the small bubbles at far left), then become ellipsoidal in the several millimeter range and can be irregularly shaped when much larger. Right: An area in the North Sea where methane is being released from the ocean floor and forms distinct bubble streams. Accurately measuring the flux, rise speed and bubble size distribution is crucial for contributing to larger scale ocean and atmosphere models. — **Left image:** Bubble shapes in water vary, being almost spherical up to a millimeter in size (the small bubbles at far left), then become ellipsoidal in the several millimeter range and can be irregularly shaped when much larger. **Right:** An area in the North Sea where methane is released from the ocean floor and forms distinct bubble streams. Accurately measuring the flux, rise speed and bubble size distribution is crucial for contributing to larger scale ocean and atmosphere models. Image courtesy of Jordt et al (2015).

Early succession of the Cinder Cones methane seep, the first active Antarctic seep to be studied. The seep is a linear feature marked by white, with sulfide-oxidizing bacterial mats on the surface. The feature was first sampled in 2012 (left panel) and continued until 2016 (center). The mat was slightly reduced in its surface area by 2014 (lower right). In 2016, more areas of active seepage were discovered, including at the 7-m (23-ft) water depth “Shallow Site” shown in the upper right. Methane gas released in shallow waters can escape the ocean and add to atmosphere warming. — Early succession of the Cinder Cones methane seep, the first active Antarctic seep studied. The seep is marked by white, with sulfide-oxidizing bacterial mats on the surface. The feature was first sampled in 2012 (left panel) and continued until 2016 (center). The mat was slightly reduced by 2014 (lower right). In 2016, more areas of active seepage were discovered, including at the 7-m (23-ft) water depth “Shallow Site” shown in the upper right. Methane gas released in shallow waters can escape the ocean and add to atmospheric warming. Image courtesy of royalsocietypublishing.org/journal/rspb Proc. R. Soc. B 287: 202011342. (CC-BY-4.0).

Below about 200 meters (650 feet), the gas bubbles dissipate or get digested by organisms with a “taste” for methane. But in shallow waters, the bubbles can burst at the surface of the sea, entering the atmosphere. Those popping bubbles matter: Methane has more than 25 times the warming impact of carbon dioxide, so these shallow-water seeps could become a serious greenhouse gas emitter — though to what degree remains hard to pin down at present.

But that’s why recently, when scientists found dozens of new cold seeps in Antarctica — some in merely 5 m (16 ft) of water — they became concerned as to how much methane might be escaping the sea into the atmosphere and whether the amount was increasing in the now warming Southern Ocean.

If the Antarctic seeps “follow the behaviour of other global seep systems, there is the potential for rapid transfer of methane to the atmosphere from a source that is not currently factored into future climate change scenarios,” Sarah Seabrook, marine scientist at Earth Sciences New Zealand, said in a statement about the recent Antarctic findings. That could mean underreporting by our climate models and underestimates for rising temperature timetables.

Methane doesn’t only bubble out of sediment seeps, the gas can also exist encased in white, icelike material on the seafloor. These gas hydrates release methane if the crystals dissociate when ocean conditions change. Argentino considers gas hydrates among the coolest things to see at methane seeps: “These pure white deposits, piercing the seafloor, are made of water molecules making crystalline structures, which host gas molecules inside. And you can see that from the ROV, and you see small chunks of it floating up and dissolving.” — Methane doesn’t only bubble out of sediment seeps, the gas can also exist encased in white, icelike material on the seafloor. These gas hydrates release methane if the crystals dissociate when ocean conditions change. Argentino considers gas hydrates among the coolest things to see at methane seeps: “These pure white deposits, piercing the seafloor, are made of water molecules making crystalline structures, which host gas molecules inside. You can see that from the ROV, as small chunks of it float up and dissolve.” Image courtesy of NOAA Ocean Exploration, Seascape Alaska.

To see a seep

The first methane seep was discovered by chance, in 1983, during a manned submersible dive in the Gulf of Mexico. Scientists later became alarmed at a significant and rapid increase in atmospheric methane around 2007 and then found major releases from thawing underwater permafrost in the Arctic off the coast of Siberia between 2008 and 2010.

But the challenge from the outset has remained how to locate and monitor such releases, since human-piloted submersibles are not an efficient strategy for finding and tracking more.

“The ocean is wide; the ocean is deep. If you want to study these ecosystems, the question is: Where to look?” said Jens Greinert, a marine geologist at GeoMar at the Helmholtz Centre for Ocean Research Kiel, in Germany. Greinert helped build a device to quantify the amount of methane released from seabed seeps.

A selection of seep sites discovered in the McMurdo Sound region of Antarctica, with year of emergence indicated. a Microbial mats (x and y) on the seafloor at 30 m (100 ft) water depth, showing depressions in the seafloor, apparent seafloor pockmarks, associated with the seep site; b A seep site at 23 m (75 ft) water depth, showing the seafloor with microbial mats and gas bubbles escaping into the water column; c–e Images of microbial mats correlated to seafloor seepage. — A selection of seep sites discovered in the McMurdo Sound region of Antarctica, with year of emergence indicated. a Microbial mats (x and y) on the seafloor at 30 m (100 ft) water depth, showing depressions in the seafloor, apparent seafloor pockmarks, associated with the seep site; b A seep site at 23 m (75 ft) water depth, showing the seafloor with microbial mats and gas bubbles escaping into the water column; **c–e** Images of microbial mats correlated to seafloor seepage. Image by Rob Robbins and Cape Barne, courtesy of Seabrook et al. (2025).

As the search for methane bubbles got underway, oceanographers first turned to echo sounders, the same acoustic technology embraced by fishers looking for catch. These sonar devices send sound waves from a ship into ocean waters. Those sounds bounce back when they hit the bottom or something else (like fish) that scatters the waves. Importantly, scientists found that methane bubbles create a characteristic flare, or plume, in the acoustic pattern, Greinert explained.

In deep-sea areas that have never been explored, ships can crisscross in a grid-like pattern to generate an acoustic map (seen, not heard) of the ocean floor. Mapping the seafloor can also reveal “pockmarks” — depressions in the ocean bottom that hint at seeps below. Geologists also make seismic maps to find areas where methane might be trapped beneath sediments.

Once a flare, or likely methane release area, is found, researchers can deploy remotely operated vehicles (ROVs) to see the seeps firsthand. These submersibles can be loaded with additional sensors, such as methane analyzers, to get detailed bubble release data. In shallow waters, scuba divers sometimes take samples from the seeps, doing so even in the icy waters of Antarctica, where the first seep was reported in 2011, in just 10 m (33 ft) of water in the Ross Sea.

A hydrophone is placed on the ocean floor to record the sound of methane bubbles. In 2016, researchers used ROV Hercules from the Ocean Exploration Trust's Exploration Vessel Nautilus to deploy a hydrophone about 10 kilometers (6 miles) off Heceta Bank on the Oregon continental margin in 1,228 m (4,028 ft) of water (about three-fourths of a mile deep). The acoustic signatures of the bubbles from the seep site are depicted in the hydrophone record as a series of short, high-frequency bursts, lasting 2-3 seconds. — A hydrophone is placed on the ocean floor to record the sound of methane bubbles. In 2016, researchers used ROV Hercules from the Ocean Exploration Trust’s Exploration Vessel Nautilus to deploy a hydrophone about 10 kilometers (6 miles) off Heceta Bank on the Oregon continental margin in 1,228 m (4,028 ft) of water (about three-fourths of a mile deep). The acoustic signatures of the seep site bubbles appear in the hydrophone record as a series of short, high-frequency bursts, lasting 2-3 seconds. Image courtesy of Ocean Exploration Trust.

Listening in

Once scientists started finding seeps, they next wanted to know how much methane they are releasing. To do that, researchers have to look at the bubbles in another way.

Methane bubbles make noise when they squeeze through seafloor sediment and escape into the water column. So scientists planted hydrophones on the ocean floor off the northwest coast of the United States.

Hydrophone recordings revealed that gas bubbles have a unique acoustic signature: a series of short, high-frequency bursts, in clusters of 2-3 seconds. The escaping bubbles sound something like bacon frying in a pan, researchers said.

By comparing these sound files with images from the ROV, scientists determined that smaller bubbles, which hold less methane, possess a higher pitch.

The next step for researchers was to refine their acoustic signature detection methods sufficiently to estimate the volume and rate of escaping methane gas, known as the methane gas flux.

The ROV SuBastian’s manipulator jaw stabilizes GEOMAR’s glowing bubble box that is capturing high-resolution images of methane bubbles rising from the seafloor. Image courtesy of ROV SuBastian/Schmidt Ocean Institute.

The genesis of the bubble box

Adding optics to the acoustic data offered another way to figure out the methane gas flux from the seafloor. “I wanted proper quantification: Numbers in kilograms or moles per time,” Greinert said. “Not guess work or distinguishing between low, middle, and high fluxes.”

Although the size and rising speed of bubbles could be calculated from the strength of the acoustic signals, the scientists realized that greater accuracy would be achieved by knowing the initial size distribution of the bubbles at the seep, explained Greinert. So a visual analysis of seep bubbles could complement acoustic data and improve the accuracy of the numbers plugged into existing equations that predicted the diffusion and dissolution of the bubbles — and whether they would ever burst at the surface of the sea.

Methane bubbles make a characteristic sound, much like frying bacon. This is an underwater recording of a methane bubble seep site at Heceta Bank in the Pacific Ocean off the coast of Oregon. Audio courtesy of Bob Dziak, NOAA/PMEL.

After several design iterations, Greinert and his colleagues created what they dubbed a “bubble box,” an instrument outfitted with stereographic cameras, recorders and “computer magic analysis” to measure the distribution of methane bubble sizes at the seafloor. With hydroacoustics to record when bubbles were released and bubble size data from the box, they could then calculate more precisely how much gas seeped out over a specific time period and which holes in the seafloor released most of the methane.

A bubble box deployed by a remotely operated vehicle in the Pacific Ocean offshore Oregon. The instrument is positioned on the seafloor over a seep spot where methane bubbles escape; bubbles rise through the instrument and are photographed by a wide baseline stereo camera setup at high frame rates for photogrammetric characterization of size, shape, rise speed and number. — A bubble box deployed by a remotely operated vehicle in the Pacific Ocean offshore Oregon. The instrument is positioned on the seafloor over a seep where methane bubbles escape; bubbles rise through the instrument and are photographed by a wide baseline stereo camera setup at high frame rates for photogrammetric characterization of size, shape, rise speed and number. Image courtesy of ROV SuBastian/Schmidt Ocean Institute.

Although the bubble box and acoustic signature studies were ready for more expeditions, by early 2020, the COVID-19 pandemic had put the world on pause. Many of the researchers have since moved on to other work, said Greinert, who now studies toxic munitions left in the sea. But learning more remains a priority. Marine seepage is currently estimated to release 20 teragrams (Tg) of methane per year into the atmosphere, representing only about 4% of annual global methane emissions. Annual methane emissions for 2024 were roughly 610 metric tons.

Total estimated atmospheric methane levels rose to 1,921.8 parts per billion (ppb) in 2024 — more than 2.6 times the preindustrial levels of 722 ppb — and those levels continue rising by sometimes record annual amounts, according to the 2025 Global Methane Budget report.

While anthropogenic emissions (from agriculture, waste management, fossil fuels and other human sources) account for about 65% of those annual emissions, the remainder comes from natural systems. The discovery of more and more methane seeps may represent a significant, unaccounted for, segment of this natural budget. The seafloor sediments potentially hold a vast methane reservoir estimated to be anywhere between 1,000 and 20,000 gigatons, which research models suggest could eventually be destabilized and released under warming conditions.

But no one expects 20,000 gigatons of methane to suddenly surge up from the seafloor sediments. Most climate models project small increases in global seafloor methane release in the next few decades, and even to the end of the century. Yet, regions like the Arctic, where temperatures are rising up to four times faster than the rest of the world, could have an outsized impact on escalating atmospheric methane levels.

“If we start decomposing the permafrost more in the Arctic, but also in the Antarctic, quicker and quicker, there might be an additional massive methane source from natural gas,” Greinert said. However, temperature is only one of many factors that determine the release of methane from seawater into the atmosphere, making it challenging to make accurate estimates.

A red cusk-eel known as congrio colorado (Genypterus chilensis), a commercially important fish highly valued in Chile, found in a tubeworm mound near a methane seep. These fish have been observed around reefs and soft-bottom mangrove ecosystems, but the importance of chemosynthetic ecosystems in sustaining their populations hasn’t been previously observed. — A red cusk-eel known as congrio colorado (*Genypterus chilensis*), a commercially important fish highly valued in Chile, found in a tubeworm mound near a methane seep. These fish have been observed around reefs and soft-bottom mangrove ecosystems, but the importance of chemosynthetic ecosystems in sustaining their populations hasn’t been previously observed. Image courtesy of ROV SuBastian/Schmidt Ocean Institute.

Beyond the bubbles

Cold seeps have attracted scientists’ attention for more than methane’s impacts on the atmosphere and climate change. Seafloor methane seeps also form unique habitats for ecosystems that support chemosynthetic creatures — methane-eating microbes, tube worms, clams, shrimp, crabs and other as-yet unnamed organisms.

“Seeps are way more common than people realize, and every seep I’ve visited has been different from every other one,” said Lisa Levin, a seep expert and marine ecologist at the Scripps Institution of Oceanography in California, who has studied seeps in the Pacific, Atlantic and Indian Oceans, some thousands of meters deep from the confines of a submersible.

Levin collaborates with geologists, microbiologists and even astrobiologists to explore these methane-infused realms. “We’re always discovering new relationships among organisms,” she said. The species’ interrelationships within these environments may extend beyond the bubbles themselves and influence the rest of the deep sea, Levin noted.

Close-up view of bacterial communities thriving at a recently discovered Arctic vent site, which scientists propose be named the Frigg Vent Field. The distance between the two green laser points is 9 centimeters (3.5 inches), providing a scale for the dense microbial structures visible on the seafloor. Image courtesy of UiT; EXTREME25; NORMAR/Ægir6000ROV.

Some seeps may even hold clues to the origins of early life. A new cold vent discovered in the Arctic is reportedly releasing methane made due to chemical reactions between water and rock under pressure. This extreme environment “could mimic processes that resemble conditions on the early Earth and possibly other ocean worlds,” expedition scientists said.

Although methane bubbles and the ecosystems they support might have a wider sphere of deep-sea influence, Levin still calls the cold seeps a “poor second cousin” to other far more studied marine habitats. Cold seeps “don’t get as much publicity and attention,” she said. “But they are pretty exceptional environments.”

The recent discovery of numerous methane seeps in Antarctica opens a new, and potentially urgent, arena for exploration by the bubble chasers. With atmospheric methane emissions now escalating at record rates — due to human activities (fossil fuel burning, landfills and livestock), along with climate change-triggered wetland and permafrost releases — the mapping of where, and how much, cold-seeps will add to the warming equation is becoming an increasingly important question demanding an answer.

Banner image: Methane bubbles flow in small streams out of seafloor sediment offshore of Virginia, north of Washington Canyon. Quill worms, anemones and patches of microbial mat can be seen in and along the periphery of the seepage area. Image courtesy of NOAA Office of Ocean Exploration and Research, 2013 ROV Shakedown and Field Trials in the U.S. Atlantic Canyons.

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A sonar image of methane plume in the Atlantic Ocean in 2014. Methane bubbles in the water column at a seep site overlooking Baltimore Canyon near the U.S. Atlantic coast are shown in green. Researcher Jens Greinert recalled his introduction, in 1998, to finding methane bubbles with echo sounders on a Russian research vessel in the Sea of Okhotsk. Image courtesy of USGS/Carolyn Ruppel/WHOI.

Measuring Methane Bubble Plumes – October 1, 2018

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