An international collaboration between researchers in Germany and China has shed new light on a long-standing mystery: hundreds of thousands of shallow depressions scattered across the floor of the North Sea. The team, whose findings appear in the science publication Contact Earth and Environment, delves into how these telltale pits form and what their presence reveals about the seafloor and its activity.
Across the North Sea, the seabed is peppered with a vast network of round or oval indentations. Some are only a few meters wide, while others reach up to around 60 meters in diameter, yet most measure a shallow depth of about 11 centimeters in any given area. For years, scientists debated the origin of these features. A popular hypothesis pointed to methane bubbling from the sediments, but this explanation encounters notable challenges under scrutiny.
Lead author Jens Schneider von Deimling, a geologist from the University of Kiel, explains that the bottom substrate of the North Sea is primarily porous sand. This composition, combined with strong bottom currents, prevents methane from collecting in the sediments. The team confirmed this assessment through extensive acoustic surveys, scanning thousands of kilometers with echo sounders and finding no evidence of gas pockets or gas-related activity in the depressions.
The researchers propose a different mechanism: the depressions were most likely created by porpoises, a marine mammal related to dolphins. The working theory is simple yet compelling. Porpoises scavenge the seabed in pursuit of prey such as sand eels. While foraging, they repeatedly push and plow along the substrate, displacing sand and leaving behind shallow pits in their wake.
This idea is supported by habitat models for both sand eels and porpoises. When scientists cross-referenced modeled eel distributions with porpoise ranges and conducted seabed surveys informed by echolocation, they observed a striking overlap: many depressions clustered in areas known to support abundant eel populations and frequented by porpoises. The convergence of these data streams strengthens the case that the pits are a byproduct of foraging rather than a natural venting feature of the seafloor.
One of the challenges the team faced was directly observing porpoises during foraging. The turbidity of the North Sea water and the animals’ shyness made it difficult to capture films or real-time footage of the hunting events. Nevertheless, the researchers emphasize that the absence of methane signals, along with the spatial correlation between depressions and predator–prey habitats, makes a dolphin-like digging mechanism a plausible explanation.
The broader significance of this discovery lies in how scientists interpret underwater methane emissions. The presence of methane can signal tectonic instability in seabed regions. In this study, the attribution of the holes to porpoises does not negate the importance of methane research, but it does recalibrate how certain seafloor features are interpreted as indicators of seismic risk. If the depressions are biogenic in origin, their existence does not imply the same level of tectonic threat as methane seepage would, providing a more nuanced view of North Sea seismic safety for offshore operations and coastal communities alike.
In a broader geographic note, researchers point to recent discoveries of massive submarine volcanic activity off the coasts of Chile and Peru. The contrast between those volcanic systems and the North Sea underscores the diversity of processes shaping seafloor landscapes around the world. While the Chile–Peru region remains a volatile hotspot, the North Sea exhibits a calmer, biologically influenced landscape with dots of shallow pits that tell a story of animal foraging rather than gas release. The study’s results encourage a more cautious and evidence-based approach to interpreting seafloor features, especially when assessing potential hazards related to gas emissions and tectonic processes.
Experts emphasize that ongoing monitoring and dedicated acoustic and ecological analyses will further clarify how widespread this porpoise-driven mechanism is across different shelf seas. The interdisciplinary approach—combining deep-sea acoustics, habitat modeling, and behavioral insights—offers a robust template for investigating similar seabed patterns elsewhere. The team stresses that understanding these microfeatures contributes to a broader comprehension of marine ecosystems, seabed dynamics, and the subtle ways biology shapes the seafloor we often take for granted. The research also highlights the value of international collaboration in unraveling ocean mysteries, where multiple viewpoints converge to illuminate how life beneath the waves interacts with the geology above.