Similar forms can conceal surprisingly different histories
Beneath the ocean's surface, thousands of circular depressions have long been read as signatures of methane escaping from the seafloor — a tidy explanation that recent science has quietly dismantled. Research converging around 2026 reveals that these formations, called pockmarks, are shaped by forces as varied as dolphins hunting buried fish, sediment slowly obeying gravity, and the ancient memory of retreating glaciers. The lesson is one the earth often teaches: identical shapes can carry entirely different histories, and the cost of a single story is measured in distorted climate data and endangered infrastructure.
- A decades-old scientific consensus — that circular seafloor craters signal methane emissions — is unraveling as researchers document radically different origins across the world's oceans.
- In the North Sea alone, more than 40,000 shallow depressions now appear to be the work of harbor porpoises churning sediment while hunting fish, not gas venting from below.
- Off California's Big Sur coast, a crater field the size of Los Angeles shows no gas signature at all — only the slow, patient pull of gravity moving sediment downhill across 280,000 years.
- In the Arctic's Barents Sea, methane does remain the sculptor, but as a relic of glacial retreat 12,000 years ago, with over 600 active gas columns still rising as evidence of that ancient rupture.
- The misattribution carries real consequences: inflated carbon emission estimates corrupt global climate models, while misread seafloor maps threaten the safety of submarine cables, pipelines, and offshore wind installations.
For decades, scientists encountering circular depressions on the ocean floor reached instinctively for the same explanation: methane gas rising through sediment and leaving behind neat round holes. The answer was tidy — and far too simple.
These formations, called pockmarks, are real and numerous, and in some places methane truly is responsible. But recent research has shown that the same circular shape can conceal entirely different stories. A 2023 study led by geoscientist Jens Schneider von Deimling at Kiel University found that more than 40,000 shallow depressions in the North Sea were almost certainly created by harbor porpoises diving to hunt fish buried in sediment. As the animals churn the seafloor, tidal currents enlarge their marks into visible craters. Direct observation has proven elusive — the water is murky and the animals wary — but the evidence is compelling.
Off Big Sur in California, a joint team from the Monterey Bay Aquarium Research Institute, the U.S. Geological Survey, and Stanford University studied a crater field covering an area the size of Los Angeles and found no meaningful trace of gas or fluids. Their conclusion: gravity, acting over at least 280,000 years, had slowly pulled sediment downslope, carving the depressions across geological time.
Methane has not left the picture entirely. In the Arctic's Barents Sea, enormous craters were torn open roughly 12,000 years ago when retreating glaciers released methane that had been locked in frozen hydrates beneath two kilometers of ice. More than 600 columns of gas bubbles still rise in the area today, evidence that the process continues.
The stakes of getting this wrong are significant. Misattributing craters to methane inflates estimates of gas emissions and distorts global carbon cycle calculations. Practically, submarine cables, pipelines, and offshore wind farms all depend on accurate knowledge of seafloor stability — and whether a depression was carved by gas, gravity, or a foraging animal determines whether infrastructure built nearby will hold. The ocean floor, it turns out, is shaped by forces far more varied than the easy answer once suggested.
For decades, scientists looking at thousands of circular depressions scattered across the ocean floor reached for the same explanation almost without thinking: methane. Gas rising through sediment, escaping into the water, leaving behind neat round holes. The explanation was tidy, obvious, and wrong—or at least, far too simple.
These formations, called pockmarks, are real enough. They dot the seafloor in staggering numbers, and in many places, methane really is the culprit. But recent research has revealed that the same circular shape can mask entirely different stories. In one location, it's a dolphin hunting for lunch. In another, it's gravity slowly pulling sediment downhill over hundreds of thousands of years. In a third, it's the aftermath of an ice age, methane locked in frozen compounds and suddenly released when the glaciers retreated. The ocean floor, it turns out, is more complicated than the easy answer suggested.
The shift in understanding came into focus around May 2026, drawing on scientific papers published over the preceding years. A study led by geocientist Jens Schneider von Deimling at Kiel University in Germany, published in 2023, examined the North Sea and found that more than 40,000 shallow depressions there had almost nothing to do with methane. Instead, they appeared to be the work of harbor porpoises—small cetaceans related to dolphins—diving to the bottom to hunt lançãos, thin fish that bury themselves in sediment. As the porpoises churn the sand searching for food, they leave small marks that tidal currents then enlarge into visible depressions. The hypothesis is strong but not yet confirmed by direct observation; the murky water and the animals' wariness have so far prevented scientists from catching them in the act.
On the opposite side of the world, off Big Sur in California, researchers from Monterey Bay Aquarium Research Institute, the U.S. Geological Survey, and Stanford University studied a field of craters covering an area the size of Los Angeles. They found no significant evidence of gas or fluids. Instead, they concluded that gravity was the sculptor. The seafloor there slopes gently downward, and sediment has been creeping slowly downhill for at least 280,000 years, with a more recent movement event about 14,000 years ago. This gradual sliding, not methane, carved the depressions over immense stretches of geological time.
Methane, though, has not been removed from the stage. In the Barents Sea in the Arctic, enormous craters were linked to the end of the last ice age. About 12,000 years ago, a layer of ice up to two kilometers thick covered the seafloor, trapping methane in the form of hydrates—solid compounds resembling ice. As the climate warmed and glaciers retreated, the gas concentrated and was released abruptly, tearing open vast depressions. More than 600 columns of gas bubbles still rise around these formations, showing that the process remains active in some areas.
Why this matters extends well beyond the satisfaction of solving a puzzle. Misidentifying the source of these craters distorts how scientists calculate the global carbon cycle. Depressions wrongly attributed to methane can inflate estimates of gas emissions that never actually occurred in those regions, skewing crucial environmental data. There is also a practical concern tied directly to infrastructure. Submarine internet cables, oil and gas pipelines, and offshore wind farms all depend on accurate maps of the ocean floor, reliable understanding of sediment stability, and knowledge of terrain movement risks. Knowing whether a crater was carved by active gas, by sliding sediment, or by a foraging animal can determine whether an offshore structure will be safe to build and operate.
The story of these circular holes in the seafloor is a lesson in how science works: an explanation that seemed obvious enough to stop questioning had to be revised in the face of new evidence. The ocean bottom, it turns out, is shaped by gas, gravity, ancient ice, and the hunger of dolphins—a reminder that similar forms can conceal surprisingly different histories, waiting for those patient enough to look closely.
Notable Quotes
The same circular shape can mask entirely different stories—methane, dolphins hunting, or gravity pulling sediment downhill— Research synthesis from multiple studies (2023-2026)
The Hearth Conversation Another angle on the story
So for decades, scientists just assumed all these holes were methane. What made them start questioning that?
High-resolution maps of the seafloor. Once they could see the formations in detail, they noticed that structurally identical-looking craters appeared in places where the geology made no sense for methane. That forced them to ask harder questions.
And they found dolphins digging? That seems almost whimsical.
It does, but it's grounded in behavior. Harbor porpoises hunt fish that hide in sediment. They dive and churn the sand. The marks they leave get enlarged by currents. It's not confirmed by direct observation yet—the water's too murky—but the evidence is strong.
What about the California site? That's a completely different mechanism.
Gravity and time. The seafloor slopes gently, and sediment has been creeping downhill for at least 280,000 years. No gas involved at all. It's a reminder that the same shape can come from radically different forces.
Does this actually change how we understand climate and carbon?
Significantly. If you misidentify the source of these craters, you overestimate methane emissions in regions where none is actually escaping. That inflates your carbon cycle calculations and distorts environmental data that policy depends on.
And there's a safety angle too?
Absolutely. Submarine cables, pipelines, offshore wind farms—they all need to know what's stable and what's moving. Knowing whether a crater came from gas, gravity, or an animal tells you whether the ground beneath your infrastructure is safe.