Fierce naval battles in the North Sea during both world wars sent many ships to the seafloor. Their wrecks remain there today, leaking toxic chemicals that are absorbed by local plants and animals.
In April 1917, the German submarine UC-30 was traveling beneath the North Sea near the Danish island of Rømø. Engine problems had forced it to abandon a mission to Ireland and head back toward Germany, putting the vessel within reach of home.
It never arrived. The submarine was carrying 18 mines and six torpedoes when, on April 19, it struck a British mine and sank. All 27 crew members died. UC-30 remained hidden on the seafloor for almost a century until Danish diver Gert Normann Andersen discovered the wreck in 2016.
The wreck later became one of the sites examined by Katrine Juul Andresen, a professor in the Department of Geoscience at Aarhus University, through the large NorthSeaWrecks research project.
Working with navy divers, Andresen obtained samples from the water and seabed around UC-30, as well as starfish and mussels living directly 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.”
TNT is already entering the ecosystem
Analysis of those samples showed that explosives still inside UC-30 are contaminating the marine environment around the wreck.
“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 Battle for the North Sea
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.
Thousands of wrecks pose wider risks
The German submarine is far from the only historic wreck contaminating Danish waters. In 2024, Katrine Juul Andresen produced a report for the Danish Environmental Protection Agency that mapped these wrecks, evaluated the environmental risks they pose, and considered possible measures to prevent or reduce 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 Danish Environmental Protection Agency commissioned the report to assess how Denmark can meet the requirements of the EU Water Framework Directive. Andresen says the findings indicate that more specific measures may eventually be necessary to address pollution from these wrecks.
“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.
Cleanup can create new environmental risks
Germany has already committed substantial funding to recover some wrecks and World War II munitions from the Baltic Sea. After the war ended, the Allies dumped tons of ammunition into the sea to keep it from falling into the wrong hands.
Like the munitions still contained in wrecks, this discarded ammunition is now contaminating 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.
But removing explosives from wrecks is not necessarily harmless. In some circumstances, detonating munitions underwater can spread contamination and make the environmental problem worse.
“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.
Andresen is working with international colleagues through the EU-funded REMARCO project to investigate methods for removing historical wrecks and their munitions.
“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 involved in the project are developing crawling robots that can move along the seabed and pinpoint dangerous munitions, allowing individual wreck sites to be monitored more precisely.
“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.”
Still, intervention may not be necessary at every wreck.
“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.
Reference: “Environmental risks from world war shipwrecks: Field-based biomarker evidence from caged mussels in the North Sea” by Romina Marietta Schuster, Franziska Isabell Binder, Lillian Tabea Hannah Bünning, Jennifer Susanne Strehse, Katrine Juul Andresen, Maarten De Rijcke, Sven Van Haelst, Uwe Wichert, Philipp Grassel, Edmund Maser and Matthias Brenner, 17 April 2026, Marine Pollution Bulletin.
DOI: 10.1016/j.marpolbul.2026.119738
The research was co-funded by the European Union (European Regional Development Fund) under the North Sea Interreg Program 2014–2020. Research was conducted within the framework of the project “North Sea Wrecks – An Opportunity for Blue Growth: Healthy Environment, Shipping, Energy Production and transmission (NSW).
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