Scientists solve mystery of millions of mysterious holes at the bottom of the ocean

A group of scientists have discovered why there are ‘millions’ of ‘crater-like’ pits on the floor of the North Sea. While many people brave swimming in the North Sea despite its freezing temperatures – cold water swimming part of your new year, new you and all that – you probably won’t have paddled out far or deep enough to have spotted a rather peculiar sight. On the sea floor of the North Sea lie ‘millions of mysterious pits’ which have been ‘scarcely investigated so far’. Well, until now that is.

A team of geoscientists from Kiel University in Germany decided to dive into the mystery of the crater-like pits on the ocean’s floor because while there are ‘thousands’ of the pits known as ‘pockmarks’, not all of them are what they first seem. The crater-like depressions are found in the sediment layer – loose sand and clay at the bottom of the water, as per EPA – and some are formed by ‘fluid discharge’ of substances such as ‘the greenhouse gas methane or groundwater’.

However, ‘the majority’ of the pits are actually ‘much shallower’ than the ‘conical craters of the pocketmarks’ and so weren’t formed in the same way as a result of a methane gas leak. So, how did they occur?

Well, with the help of modern multibeam echosounder technology, the team from Kiel University found a whopping 42,458 crater-like pits during their mission – add that to your list of rogue sharks or slimy octopus tentacles to worry about when you next glance down while taking a dip in the North Sea.

Oh, and the team believe there’s a lot more than that too, those are just the ones they managed to count in the specific area they conducted their investigation in. They reckon there are probably millions of them around the world.

The pits are ‘enigmatically shaped’ and ‘shallow’ with ‘an average depth of just eleven centimeters,’ according to a press release by Kiel University.

And how they’re formed? Well, lead author on the study, Dr Jens Schneider von Deimling says: “Our results show for the first time that these depressions occur in direct connection with the habitat and behavior of porpoises and sand eels, and are not formed by rising fluids. “Our high-resolution data provide a new interpretation for the formation of tens of thousands of pits on the North Sea seafloor, and we predict that the underlying mechanisms occur globally, but have been overseen until now.” But what on earth are the porpoises and sand eels getting up to to create such crater-like shapes?

The crater-like pits are formed as a result of the sea animals searching for food – sadly, sand eels are also themselves ‘an important food source for the North Sea population’ including porpoises, Dr Anita Gilles of the TiHo-Institute for Terrestrial and Aquatic Wildlife Research (ITAW) adds. Von Deimling resolves: “We had to come up with an alternative hypothesis for the formation. This allowed us to predict where potential porpoise feeding sites are, and that is exactly where we found the pits – always close to sand eel habitats.

“Our extensive and multidisciplinary data analysis now provides a conclusive explanation for our harbor porpoise pits hypothesis.”

This discovery is so important because it means scientists can now be more active in protecting sea life from renewable energy sectors building offshore structures on top of the pit areas, which would destroy the marine environment underneath.

And this discovery doesn’t just affect the area of sea the team focused on, with the crater-like pits expected to be found all over the globe too. The study – titled Millions of seafloor pits, not pockmarks, induced by vertebrates in the North Sea – was published in the Communications Earth and Environment journal in December 2023.


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