A German World War I submarine that disappeared off the coast of Denmark more than a century ago is still harming the surrounding environment — not with torpedoes, but with toxic chemicals.

Researchers have said TNT is seeping from the wreck of UC-30 into the North Sea, prompting fresh worries about marine ecosystems, seafood networks, and the coastal communities tied to them.

Here’s what to know

According to Gizmodo, UC-30 was lost in 1917 near Rømø when it struck a British mine. On board were 18 mines, six torpedoes, and 27 crew members.

For nearly a century, the wreck went unidentified. It was later recognized during a wider project to chart wartime wrecks in Danish waters. 

To evaluate pollution around the site, Aarhus University geoscientist Katrine Juul Andresen and her co-authors reported in the journal Marine Pollution Bulletin that they used blue mussels as indicators of contamination. Their analysis found TNT in the nearby water, seabed, and marine life.

“Pollution is definitely occurring,” Andresen said in a statement cited by Gizmodo. “We found traces of TNT in the wildlife, the seabed, and the water column.”

Because the North Sea supports both commercial fisheries and interconnected food webs, chemicals leaking from old munitions could spread through ecosystems and into seafood supplies.

More background

The researchers found TNT concentrations in seawater near UC-30 of about 274.8 nanograms per gallon. Even at that low level, the mussels showed signs of biological stress, including trouble with detoxification, disrupted metabolism, and reduced glycogen in digestive tissue — an indicator tied to the energy reserves needed for reproduction.

As Gizmodo reported, the team also studied areas near two other wrecks — the World War I cruiser SMS Mainz and the World War II transport KW 58 Hendricus. Andresen said the full scale of the problem will require much more study, especially because so many wrecks have not been closely examined.

In a 2024 report cited by Gizmodo, Andresen wrote that Danish waters contain 10,127 wrecks and that roughly 15% date to the eras surrounding the two world wars. That suggests UC-30 is likely not an isolated threat, but part of a much larger legacy problem resting on the seafloor.

Study co-author Edmund Maser, a toxicologist based at the University Medical Center Schleswig-Holstein in Kiel, told the American Geophysical Union’s Eos, via Gizmodo, “The problem with these ammunition compounds is that they are not only toxic but also carcinogenic.”

What’s being done?

A central task is determining which wrecks pose the greatest danger and documenting the substances they are releasing. Andresen and her collaborators are contributing to that evidence base, which could help governments make more targeted cleanup choices.

Not every wreck should be handled the same way, however. Andresen noted that some sunken vessels are fully or partly buried under sand, which can reduce the risk of pollutants spreading into surrounding waters. In those cases, disturbing the site could create new hazards instead of solving them.

Other options are more difficult and expensive. Andresen pointed to approaches ranging from robotic recovery operations to removing or neutralizing munitions. Germany has already committed significant resources to retrieving similar explosives from the Baltic Sea, showing that action is possible, but costly.

“It is a matter of identifying where the environmental threat is greatest — and then deciding whether the munitions on the wrecks should be removed,” Andresen said, per Gizmodo.

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