In April 1917, the German submarine UC-30 sailed beneath the waves off the coast of Rømø. It had originally been on a mission in Ireland, but after suffering engine trouble, it turned around and was almost home in Germany.
Onboard, the submarine carried 18 mines and six torpedoes, making it heavily loaded with explosives. However, the submarine never made it back home. On April 19, it struck a British mine and sank. Twenty-seven crew members perished, and the wreck vanished until it was discovered in 2016, nearly 100 years later, by Danish diver Gert Normann Andersen.
The submarine is one of the wrecks that Katrine Juul Andresen, a professor at the Department of Geoscience at Aarhus University, helped investigate as part of the major research project NorthSeaWrecks .
Together with navy divers, she collected samples of the surrounding water, the seabed, and the starfish and mussels living on the wreck.
“Civilian diving is not permitted on the wreck, so we received great assistance from navy divers. After receiving instructions, they dove down and collected sediment samples, water samples, and wildlife samples from various locations on the wreck and from the surrounding seabed.”
“The divers also recorded video and took photos, which we subsequently used to assess the exposure of the mines and the condition of the wreck.”
After analyzing the samples, the professor concluded that the explosives inside the wreck are polluting the surrounding ecosystem.
“We found traces of TNT in the wildlife, the seabed, and the water column. Pollution is definitely occurring. The question is how significant the effect is and how far from the wreck the pollution impacts the marine environment. It is likely a relatively localized source of pollution, but it will worsen as the wreck continues to deteriorate and expose more of the explosive material inside the mines,” she says.
The North Sea was critical during World War I for both Germany and Britain. Whoever controlled the waters could dictate which goods entered and exited both countries, effectively allowing them to starve the opponent into submission.
Germany, which had a smaller fleet than Britain, attempted from the beginning of the war to use its submarines to sneak up on British ships and sink them.
Both nations also laid thousands of mines, hoping enemy ships would hit them. An estimated 190,000 mines were deployed in the North Sea during WWI.
Additionally, the largest naval clash in history took place in the North Sea off Hanstholm in Northern Jutland during WWI. The German and British navies clashed at the Battle of Jutland, where more than 25 ships sank and nearly 10,000 sailors lost their lives. As a result, numerous wrecks from that era rest on the seabed.
One of Many Wrecks
The German submarine is only one of many historical wrecks polluting Danish waters. In 2024, Katrine Juul Andresen prepared a report for the Danish Environmental Protection Agency to map and assess the environmental risks posed by such wrecks – and to determine potential measures to prevent or mitigate the pollution.
“In the report, I found that there are 10,127 wrecks in Danish waters. Around 15 percent of them date back to the years around and during the two world wars [1910–1920 and 1940–1945, ed.]. That is quite a high number,” she says.
The report was commissioned by the Danish Environmental Protection Agency to evaluate how Denmark can comply with the EU Water Framework Directive. Consequently, the professor suggests that more concrete measures may eventually be needed to manage this type of pollution.
“I am not aware of whether the agency has taken the work further since the report was published in 2024. However, research shows that the wrecks represent a source of pollution for the local marine environment,” she says.
Detonate or Retrieve?
In Germany, lawmakers have allocated substantial funds to retrieve part of the wrecks and WWII munitions from the Baltic Sea. When the second world war ended, the Allies dumped tons of ammunition into the sea to prevent it from falling into the wrong hands.
Just like the wrecks, however, this submerged ammunition is polluting the marine environment.
“The Germans are further ahead with this than we are in Denmark. They sometimes retrieve explosives from the wrecks and neutralize them, but doing so is very costly,” she notes.
The issue with wrecks loaded with munitions is that detonating them underwater can, in certain cases, disperse the contamination and worsen the problem.
“It depends on the area where the wreck and munitions lie. Currents and seabed morphology play a major role. In some cases, detonating them can do more harm than good ,” she explains.
Robots on the Seabed
Alongside international colleagues, Katrine Juul Andresen participates in the EU-funded REMARCO project, researching methods to remove munitions and historical wrecks.
“We are trying to identify the most gentle solutions to the problem, as there is unfortunately no easy way to clean it up,” she says.
Some researchers within the project are experimenting with developing crawling seabed robots capable of mapping the precise locations of dangerous munitions, allowing for better monitoring of the wrecks.
“In some cases, the robots can also pick up items, retrieve them from the sea and neutralize them. That could be one viable solution for the future.”
However, not all wrecks require intervention.
“Some wrecks are fully or partially buried in the seabed. When covered by sand, the risk of polluting the surrounding sea is significantly lower. It is a matter of identifying where the environmental threat is greatest – and then deciding whether the munitions on the wrecks should be removed,” she concludes.
Marine Pollution Bulletin
10.1016/j.marpolbul.2026.119738
Experimental study
Not applicable
Environmental risks from world war shipwrecks: Field-based biomarker evidence from caged mussels in the North Sea
3-Aug-2026
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.