An estimated 190,000 mines were laid across the North Sea during the war. The region also hosted the 1916 Battle of Jutland off Hanstholm, the largest naval clash in history, in which more than 25 ships were sunk and nearly 10,000 sailors died.

Those losses left the seabed strewn with wrecks, many of them still carrying munitions.

How widespread is the problem?

UC-30 represents only a fraction of the underwater hazards in Danish waters. A 2024 report Andresen prepared for the Danish Environmental Protection Agency cataloged 10,127 wrecks in Danish waters, with about 15 percent dating to the two world war periods (1910-1920 and 1940-1945).

The agency commissioned the work as part of its effort to meet EU Water Framework Directive requirements.

"I am not aware of whether the agency has taken the work further since the report was published in 2024," Andresen said. "However, research shows that the wrecks represent a source of pollution for the local marine environment."

Why is cleanup so difficult?

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Germany has already committed substantial funding to recovering munitions and wrecks from the Baltic Sea, where the Allies dumped tons of post-World War II ammunition to keep it out of enemy hands. Those weapons now form a second, larger source of marine contamination.

Andresen noted that German teams sometimes retrieve and neutralize explosives from wrecks, but the work is expensive.

Detonating munitions in place is not always safer. Underwater blasts can spread contaminants, and the result varies with local conditions.

"It depends on the area where the wreck and munitions lie," she explained. "Currents and seabed morphology play a major role. In some cases, detonating them can do more harm than good."

Could robots help?

Through the EU-funded REMARCO project, Andresen is collaborating with international researchers on gentler approaches to submerged munitions. One idea under testing uses seafloor-crawling robots that can map munitions precisely and track how wrecks break down over time.

Some of the robots may eventually retrieve items from the seabed for safe disposal.

"In some cases, the robots can also pick up items, retrieve them from the sea and neutralize them," Andresen said. "That could be one viable solution for the future."

Still, she stressed that removing material from every wreck would be neither practical nor necessary. Many wrecks sit partly or fully buried under sediment, which significantly slows the release of pollutants.

"When covered by sand, the risk of polluting the surrounding sea is significantly lower," she said. "It is a matter of identifying where the environmental threat is greatest — and then deciding whether the munitions on the wrecks should be removed."

The findings appear in Marine Pollution Bulletin (DOI: 10.1016/j.marpolbul.2026.119738).