Decode millions of mysterious holes at the bottom of the North Sea

Holes ranging from a few meters to 60 meters wide scattered across the seabed off the coast of Germany were initially thought to be formed by methane gas, but were actually created by dolphins.

Under the choppy waters of the North Sea, shallow holes dot the seabed . These holes are round or oval with a width from a few meters to more than 60m, but only 11m deep. Some craters even merge with each other, creating an aggregate-like depression. Such shallow holes often form when liquid containing methane or other groundwater bubbles up from sediment. But research published in the journal Communications Earth & Environment shows that thousands, or even millions, of holes in the North Sea could be the result of dolphin feeding . Research results show that dolphins and many other animals may play a large role in shaping the seabed, Live Science reported on February 27.

Shallow holes at the bottom of the North Sea were initially thought to be caused by leaking methane gas. (Photo: Jens Schneider von Deimling).

For years, geoscientist Jens Schneider von Deimling at Kiel University wondered whether the shallow hole in the North Sea was caused by leaking methane gas. The bottom of the North Sea is made of loose sand and has strong ocean currents, which are not suitable for methane gas to accumulate in the sediment layer. Mapping studies using echo sounders did not detect methane gas.

To better understand the mysterious shallow holes, the research team used a multi-beam echosounder, which allows surveying the seabed at high resolution. The new tool helps researchers examine the hole's shape in centimeter-sized detail. Multi-beam echosounders revealed that in fact, the shallow hole is not cone-shaped as in the case of methane gas released through sediment, according to Schneider von Deimling. Regardless of width, the holes are approximately 11 cm deep.

When searching for the cause of the shallow hole, Schneider von Deimling consulted a friend who was a biologist and diver. Thanks to that, he knew that mouse dolphins ( Phocoena phocoena ) often scour the seabed to sniff out sand eels . After the call, Schneider von Deimling collaborated with biologists studying dolphins.

The research team used existing models to predict the habitat of rat dolphins and sand eels along with data on ocean currents. Both the rat dolphin and the sand eel live in areas with strong ocean currents. The researchers found that their habitat overlapped with the study area. Wherever they expected to find dolphins and sand eels, they found more holes. The large holes were created by dolphins and eroded by ocean currents.

Currently, the team is working with scientists in Ireland to confirm their prediction of the hole's location based on the dolphin's habitat in the North Sea. Interdisciplinary research of this type can help biologists learn more about animal behavior. Understanding how shallow holes in the seafloor form is important for identifying underwater hazards. Holes created by leaking methane gas could be a sign of a threat from tectonic plates. If scientists recognize that the crater was created by living organisms, they may be able to quell concerns about tectonic activity.