Pockmarks are crater-like depressions in the seafloor, formed when gas, groundwater, or other fluids push upward through soft sediment and blow it away. First described off the coast of Nova Scotia in the late 1960s, they have since been found in oceans, seas, and even freshwater lakes worldwide, ranging from a few meters across to over a kilometer in diameter. Their abundance is striking: a single survey area in the southeastern North Sea documented more than 50,000 of them in water shallower than 50 meters.1Geochemistry, Geophysics, Geosystems. The Enigmatic Pockmarks of the Sandy Southeastern North Sea Far from being mere curiosities of the ocean floor, pockmarks serve as windows into what is happening beneath the seabed and, increasingly, into how Earth’s climate system interacts with buried carbon.
How Gas Builds Up and Breaks Through
The most common explanation for pockmark formation involves gas, usually methane, accumulating beneath a fine-grained sediment layer that acts as a cap or seal. As gas pressure builds, it eventually overcomes the seal’s resistance. Laboratory experiments modeling this process have identified several ways the breakthrough happens: the seal can dome upward and crack; faults can form around a plug of sediment that gets pushed up; or, in softer and thicker seals, individual gas bubbles simply work their way through the material. Which mode dominates depends on how stiff and thick the seal is. Repeated episodes of gas escape suspend sediment at the surface and gradually carve out the characteristic depression.2Journal of Geophysical Research: Solid Earth. Gas Seepage and Pockmark Formation From Subsurface Reservoirs: Insights From Table‐Top Experiments
An earlier theoretical framework describes the process in even more vivid terms. Gas trapped below a capillary seal accumulates until the seal fails completely, releasing a large volume of gas into an upward-rushing chimney. This chimney displaces water like a piston as it rises. Near the seafloor, the upward water flow suspends the sediment grains so they lose contact with one another and become what engineers call “quick,” essentially liquefied. Ocean-bottom currents then carry the loosened sediment away, leaving the pockmark behind.3Marine and Petroleum Geology. The physics of gas chimney and pockmark formation, with implications for assessment of seafloor hazards and gas sequestration The result can be dramatic: a single event can excavate a circular pit tens of meters deep and hundreds of meters wide.
Groundwater Can Do It Too
Gas is not the only fluid responsible. In some coastal settings, fresh groundwater flowing from onshore aquifer systems pushes up through the seabed with enough force to form pockmarks. In Puck Bay, on the southern Baltic Sea coast, researchers traced pockmark formation to groundwater moving through a system of aquifers extending from land beneath the bay floor. Chemical profiles of the pore water inside these pockmarks confirmed upward flow of freshened groundwater, and the rates of submarine groundwater discharge varied considerably from one pockmark to the next.4PubMed. Pockmarks and associated fresh submarine groundwater discharge in the seafloor of Puck Bay, southern Baltic Sea
Eckernförde Bay in the southwestern Baltic Sea offers an intriguing twist: researchers found smaller pockmarks, typically less than 15 meters across, sitting inside much larger, previously mapped pockmarks. These nested features, dubbed “intrapockmarks,” formed where gas and groundwater ascend together through localized pathways. Their acoustic signatures helped scientists identify them as a distinct category of “eyed” pockmarks, where the combination of organic-rich muddy sediment, shallow free gas, and active groundwater flow creates layered fluid-escape features.5Geochemistry, Geophysics, Geosystems. Complex Eyed Pockmarks and Submarine Groundwater Discharge Revealed by Acoustic Data and Sediment Cores in Eckernförde Bay, SW Baltic Sea
How Big Do They Get
Pockmarks span an enormous range of sizes. In the North Yellow Sea Basin, multibeam surveys mapped pockmark fields covering more than 2,000 square kilometers. Individual pockmarks there averaged about 1.36 km along their long axis and 0.78 km along the short axis, with shapes including circular, elliptical, linear, polygonal, and irregular forms.6Marine Geology. Acoustic and biological characteristics of seafloor depressions in the North Yellow Sea Basin of China: Active fluid seepage in shallow water seafloor These are among the larger examples. At the other end of the spectrum, some pockmarks in sandy continental shelves are just a few meters across and sit in water barely 50 meters deep.
In the North Sea’s Witch Ground Basin, scientists classified pockmarks into distinct size classes. The largest were more than 6 meters deep, over 250 meters long, and more than 75 meters wide, and they showed signs of active fluid venting connected to deep methane sources through vertical conduits in the subsurface.7Geochemistry, Geophysics, Geosystems. Pockmarks in the Witch Ground Basin, Central North Sea Smaller pockmarks in the same basin appeared to be relict features, no longer actively seeping. Size alone does not tell you whether a pockmark is alive or dead, but deeper, steeper-walled pockmarks sitting above clear subsurface plumbing are more likely to still be channeling fluids.
Pockmarks in Lakes
Pockmarks are not confined to the ocean. Lake Constance, the second-largest prealpine lake in Europe, hosts about 500 pockmarks in its eastern basin, with diameters reaching 16 meters. Roughly a quarter of them continuously release methane as visible bubbles. Isotopic analysis showed the methane was biogenic, produced by microbial breakdown of organic matter in the sediment. Pore-water measurements inside the shallow-water pockmarks revealed methane concentrations above 1,100 micromoles per liter, with diffusive methane fluxes and oxidation rates far higher than in surrounding sediments.8Limnology and Oceanography. Active pockmarks in a large lake (Lake Constance, Germany): Effects on methane distribution and turnover in the sediment
Lake Neuchâtel in Switzerland holds some of the largest known lacustrine pockmarks, reaching up to 160 meters across and 30 meters deep. Inside one of these pockmarks, researchers found roughly 60 meters of suspended sediment. The water within the suspension was about 2.6 degrees Celsius warmer and isotopically lighter than the surrounding lake water, confirming active inflow of karstic groundwater from the nearby Jura Mountains. The levees around the pockmarks contained deposits of mobilized subsurface sediment, recording past episodes of violent fluid expulsion.9Geophysical Research Letters. Giant lacustrine pockmarks with subaqueous groundwater discharge and subsurface sediment mobilization These lake systems remind us that the processes making pockmarks are universal wherever fluids find their way upward through soft sediment, regardless of whether the water above is salty.
Biological Hotspots on the Deep Seafloor
The deep ocean floor is generally sparse in life, so pockmarks that actively seep methane stand out as biological oases. At Vestnesa Ridge off Svalbard, near 79°N, researchers found that infaunal biomass and abundance inside methane-rich pockmarks were five times higher than in a nearby control area. Species richness was two and a half times higher, and diversity about one and a half times higher. Microbial mats, chemosymbiotic worms, and carbonate outcrops dominated the pockmark interiors, while predators clustered around these features. The methane essentially fuels a food web from the bottom up: microbes use it as an energy source, supporting communities of specialized invertebrates, which in turn attract more conventional deep-sea animals.10Limnology and Oceanography. Methane cold seeps as biological oases in the high‐Arctic deep sea
Similar patterns appear at lower latitudes. A giant pockmark roughly 800 meters across at about 3,160 meters depth along the Congo-Angola margin hosted dense assemblages of mussels, clams, and tubeworms. Isotopic analysis of their tissues confirmed that the mussels derived nutrition from methane through symbiotic bacteria, while the clams and tubeworms depended on hydrogen sulfide, another by-product of seep chemistry.11Marine Ecology. Cold‐seep assemblages on a giant pockmark off West Africa: spatial patterns and environmental control The most active central zone of this pockmark, which sat in a morphological depression with abundant carbonate rubble and high methane fluxes, supported the densest clusters of mussels and tubeworms.12Deep Sea Research Part II: Topical Studies in Oceanography. Influence of seep emission on the non-symbiont-bearing fauna and vagrant species at an active giant pockmark in the Gulf of Guinea (Congo–Angola margin) The zonation from center to edge to surrounding seafloor creates a gradient of habitats that collectively boosts regional biodiversity in what would otherwise be a featureless abyssal plain.
Carbonate Crusts as a Chemical Archive
Where methane seeps through seafloor sediment for long periods, a distinctive type of rock forms. Microbes in the sediment consume methane by coupling it with sulfate from the seawater, a process that raises the alkalinity of the pore water and causes calcium carbonate to precipitate. The resulting “authigenic carbonates” come in various forms: crusts, slabs, nodules, and tubular structures. Their carbon isotope signatures are extremely negative, reflecting the biogenic methane that provided their carbon. At the Kouilou pockmarks on the Congo deep-sea fan, at about 3,100 meters depth, researchers recovered nodules of high-magnesium calcite alongside aragonitic crusts, each produced by different groups of methane-consuming archaea working at different depths within the sediment.13Marine Geology. Patterns of carbonate authigenesis at the Kouilou pockmarks on the Congo deep-sea fan
Similar carbonates in the Bay of Bengal showed carbon isotope values as low as about negative 58 per mil, firmly pointing to biogenic methane as the carbon source. Enhanced pore-water alkalinity driving the carbonate precipitation was attributed to microbially mediated anaerobic oxidation of methane.14Geochemistry, Geophysics, Geosystems. Biogeochemical Reconstruction of Authigenic Carbonate Deposits at Methane Seep Site off Krishna‐Godavari (K‐G) Basin, Bay of Bengal At the Nyegga complex in the Norwegian Sea, bulk carbonate cements yielded carbon isotope values as low as negative 52 per mil and oxygen isotope values elevated enough to suggest formation near the seafloor under cold conditions, consistent with methane-driven precipitation.15Marine Geology. Comparison and implications from strikingly different authigenic carbonates in a Nyegga complex pockmark, G11, Norwegian Sea These carbonate deposits are useful well beyond their chemistry. Because they can be dated, they serve as timestamps for past episodes of methane seepage, connecting pockmark activity to specific periods in Earth’s climate history.
The Methane Hydrate Connection
A substantial share of the world’s methane is locked in gas hydrates, ice-like solids that form under high pressure and low temperature in marine sediments. When ocean temperatures rise, the stability zone where hydrates can exist shifts, and some hydrates break down. The freed methane can migrate upward and vent through pockmarks. Off the coast of the southern hemisphere, researchers interpreted methane release through a pockmark field at 520 to 660 meters water depth as a consequence of recent ocean warming destabilizing the edge of a formerly stable hydrate system.16Nature Communications. Gas hydrate dissociation linked to contemporary ocean warming in the southern hemisphere
The distances methane can travel underground before reaching the seafloor are sometimes remarkable. On the Mauritanian margin, methane migrated at least 40 kilometers below the base of the hydrate stability zone and vented through 23 pockmarks at the shelf break, probably during warmer interglacial periods of the Quaternary. Thin sand or silt layers provided migration routes for the gas. The venting likely occurred episodically from the middle Quaternary and possibly continues today.17Nature Geoscience. Long-distance migration and venting of methane from the base of the hydrate stability zone These findings challenge the assumption that deep-water hydrate methane stays safely locked away. Under the right geological plumbing, it can find its way to the ocean, and pockmarks are the surface expression of that leakage.
The sheer volumes involved occasionally reach eye-catching scales. Off New Zealand, researchers estimated that if methane from a single blowout event at one of the large pockmarks (8 to 11 kilometers across) reached the atmosphere, it would be equivalent to roughly 3 percent of the current annual global methane released from all natural sources.18Geophysical Research Letters. Gas escape features off New Zealand: Evidence of massive release of methane from hydrates In practice, most methane released at the seafloor dissolves in the water column or is consumed by bacteria long before it reaches the surface, so the direct atmospheric impact of any single pockmark event is uncertain. But cumulatively, seep systems across continental margins represent a non-trivial part of the ocean methane budget, and understanding pockmarks helps quantify it.
Reading Earth’s Climate History Through Pockmarks
Because pockmark activity responds to changes in temperature, ice loading, and sea level, the sediment records inside them can be read like a timeline of past environmental shifts. At Vestnesa Ridge, discrete layers of bivalve shells in pockmark sediment cores marked horizons of sustained methane seeping. Negative carbon isotope values in planktonic fossils, along with methane-derived carbonate concretions and pyrite-encrusted fossil worm tubes, confirmed that each shell layer corresponded to a paleo-seep environment.19Geochemistry, Geophysics, Geosystems. Bivalve shell horizons in seafloor pockmarks of the last glacial‐interglacial transition: a thousand years of methane emissions in the Arctic Ocean
The giant Troll pockmark field in the northern North Sea provides another case study. Uranium-thorium dating of carbonate blocks recovered from its pockmarks yielded ages of about 9,590 years, placing their formation in the early Holocene, just after the last ice age. Radiocarbon dating of microfossils in the pockmark sediments agreed with those ages. The implication is that the pockmarks formed during a specific climate transition, when warming and changing pressure conditions triggered gas hydrate dissociation and a pulse of fluid venting.20Earth and Planetary Science Letters. A climatic trigger for the giant Troll pockmark field in the northern North Sea At the Nyegga pockmark field on the Norwegian margin, the timing of maximum gas hydrate breakdown correlated tightly with peak pockmark activity, and both were linked to rapid sedimentation from glacial processes after the Last Glacial Maximum.21Nature Communications. Glacigenic sedimentation pulses triggered post-glacial gas hydrate dissociation These overlapping lines of evidence turn pockmark fields into geological archives, recording exactly when and how aggressively methane escaped during past warming events.
Risks for Seabed Infrastructure
Engineers have worried about pockmarks since the early expansion of offshore oil and gas infrastructure. A pockmark represents weakened or absent sediment in the very spot where a pipeline, cable, or platform foundation might need to sit. Active pockmarks add the complication of ongoing fluid seepage, which can reduce the bearing strength of surrounding sediment. The concern has been flagged since at least the late 1980s, when researchers explicitly evaluated the relevance of pockmarks to seabed construction.22Quarterly Journal of Engineering Geology. The formation of pockmarks and their potential influence on offshore construction Today, standard practice for offshore development in pockmark-prone regions includes high-resolution bathymetric surveys to map their extent and subsurface seismic profiling to check for active gas conduits below.
The hazard is not limited to structural instability. If gas accumulates beneath a sealed seafloor and breaks through suddenly, the resulting eruption can liquefy a patch of sediment in seconds, as described in the gas chimney model. For pipeline routes crossing areas with known subsurface gas, the possibility of a blowout event that momentarily removes the seabed’s load-bearing capacity is a real engineering scenario. Routing infrastructure around pockmark clusters, or at minimum understanding which ones are connected to active deep gas sources, is now a standard part of offshore geohazard assessment.
When Fish Make the Pockmarks
Not every seafloor pit is carved by escaping fluid. Off northwestern Australia, within the Stag oil field, researchers examined small pockmarks and proposed a very different origin. ROV footage showed large grouper species repeatedly scooping sediment from pit centers with their mouths and ejecting it over nearby mounds. The researchers concluded that the pockmarks were essentially fish-dug nests or feeding pits, not products of gas or groundwater seepage at all.23Marine and Petroleum Geology. Evidence for the biotic origin of seabed pockmarks on the Australian continental shelf This is a useful reminder that the word “pockmark” describes a morphology, a depression in the seabed, rather than a single process. When surveys encounter small, shallow pockmarks in areas without obvious subsurface gas signatures, biological excavation is worth considering alongside the standard geological explanations.
Pockmarks as Refuges From Bottom Trawling
In heavily fished regions, pockmarks may serve an unexpected conservation role. Because their walls and interiors are difficult for trawl nets to sweep through, they can shelter organisms that would otherwise be destroyed by commercial bottom-contact fishing. In one study, large, slow-growing gorgonian corals, species that are highly vulnerable to physical disturbance, were found thriving in the centers of pockmarks in an area where fishing records showed intensive trawling.24Limnology and Oceanography. Pockmarks: Refuges for marine benthic biodiversity Habitat enrichment from the seep-related chemistry likely contributed, but the physical protection of the depression itself was considered a key factor. In areas where the surrounding seafloor has been flattened and impoverished by repeated trawling, pockmarks may function as de facto marine reserves, preserving patches of biodiversity that can potentially seed recovery in adjacent habitats.