The remains of a German submarine that sank off Denmark more than a hundred years ago continue to release TNT into the ocean, and experts say the issue may grow worse. The vessel UC-30 went down in 1917 following a collision with a British mine, and its explosives are now seeping into the surrounding seawater, seafloor, and marine life.
The NorthSeaWrecks research project was led by Katrine Juul Andresen, who teaches at the Department of Geoscience at Aarhus University. She coordinated with navy divers to gather water, sediment, starfish, and mussels from around the wreck. Her team discovered signs of TNT in all three settings.
The Sinking of UC-30
UC-30 was making its way through waters near Rømø in April 1917. It had abandoned a mission to Ireland because of engine problems and was turning back toward Germany. The vessel carried 18 mines and six torpedoes, leaving it packed with explosives. On April 19, it struck a British mine and sank. All 27 crew members died.
For close to a century, the wreck lay hidden beneath the waves. Then, Danish diver Gert Normann Andersen found it in 2016.
What the Samples Showed
The wreck and its immediate surroundings were sampled by Andresen and her team. Divers captured video and images of the mines, documenting their state.
“We found traces of TNT in the wildlife, the seabed, and the water column. Pollution is definitely occurring,” Andresen says. “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.”
The Battle for the North Sea
During World War I, command of the North Sea held great strategic value. The two main powers involved, Germany and Britain, both depended greatly on naval mines. Over the course of the conflict, an estimated 190,000 mines were laid in the North Sea.
Off Hanstholm in Northern Jutland, the Battle of Jutland was fought, and it claimed more than 25 ships and nearly 10,000 sailors. The war’s toll on the North Sea seabed was great, leaving wrecks scattered across it that still remain today.
The Scale of the Problem in Danish Waters
Andresen’s report for the Danish Environmental Protection Agency, prepared in 2024, addresses the larger problem. It examines the environmental risks from historical wrecks in Danish waters. The report estimates that there are 10,127 such wrecks in those waters, with roughly 15 percent originating from the years surrounding and encompassing both world wars.
As part of its effort to figure out how Denmark can meet the requirements of the EU Water Framework Directive, the Danish Environmental Protection Agency asked for the report. Andresen says she does not know whether the agency has moved forward with the work since the report came out in 2024. Research shows that the wrecks represent a source of pollution for the local marine environment.
Why Removing Explosives Is Hard
The Baltic Sea holds a legacy of wartime waste, with Germany having already committed significant funding toward recovering wrecks and WWII munitions from its waters. The Allies disposed of vast quantities of ammunition after the war by dumping it into the ocean, and decades later, those weapons have emerged as another source of marine contamination.
Andresen points out that “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,”.
It isn’t always the safest approach to destroy explosives where they sit. Setting off a charge beneath the water sends its force outward, carrying whatever it has broken apart into the surrounding area. In certain places, doing so might actually increase pollution instead of fixing the problem.
The markers themselves, like “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,” are not part of the text; they are merely placeholders for quotations and figures that have been extracted from their original context. Each marker points to a specific passage or figure that has been lifted out, and every single one of them appears exactly once in the document.
Robots and Gentle Solutions
The EU-funded REMARCO project, which is investigating safer methods for handling submerged munitions and historic wrecks, is also involving Andresen and other international researchers.
“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.
Researchers are testing machines that can study sunken ships and chart their buried weapons without touching them.
The Wider Picture
A single sunken ship, leaking explosives for over a century, stands as proof of what can happen when a vessel sinks and no one returns to retrieve it. That solitary wreck serves as a warning sign. The North Sea conceals thousands more vessels, and all of them are decaying.
Andresen’s report, drawn up in 2024, took stock of the scale of the problem. It is not yet known whether Denmark will act on it. Explosives are already leaking from the wreck, which continues to fall apart.
A gentler approach exists for the wreck, one that involves sending robots to survey it rather than blowing it apart. Yet these surveys are not a solution. They are merely a means of watching the problem unfold as the ruin keeps falling apart.
The TNT is in motion, and Andresen’s team has proven it. The open question is whether any party will step forward to halt its progress.
Key Facts Box
- UC-30 sank April 19, 1917
- Discovered by Gert Normann Andersen in 2016
- 10,127 wrecks in Danish waters
- Around 15 percent date to 1910-1920 and 1940-1945
- Approximately 190,000 mines placed in the North Sea during WWI
- Nearly 10,000 sailors died in the Battle of Jutland
- EU-funded REMARCO project exploring safer wreck disposal methods
UC-30 was a vessel that met an unhappy end, and its crew perished with it. Nobody attempted to bring it back after it sank, and now, a century later, it is poisoning the sea.
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