In April 1917, the German submarine UC-30 was making its way through waters near Rømø after abandoning a mission to Ireland because of engine problems. The vessel had turned back toward Germany and was close to reaching home.
UC-30 was carrying 18 mines and six torpedoes, leaving the submarine packed with explosives. On April 19, it struck a British mine and sank. All 27 crew members died.
For nearly a century, the wreck remained lost beneath the sea. Danish diver Gert Normann Andersen finally discovered it in 2016.
UC-30 later became one of the wrecks examined through the major NorthSeaWrecks research project. Katrine Juul Andresen, a professor at the Department of Geoscience at Aarhus University, helped investigate the site.
Working with navy divers, Andresen and her colleagues collected water and sediment from around the wreck. They also sampled starfish and mussels living on or near the submarine.
"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."
The results showed that explosive material from the wreck is entering the surrounding marine environment.
"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.
During World War I, control of the North Sea was strategically vital to both Germany and Britain. Dominating the region meant controlling the movement of supplies into and out of each country, giving either side the ability to place enormous economic and military pressure on its opponent.
Germany had a smaller surface fleet than Britain, so submarines became an important part of its strategy. German submarines attempted to approach British vessels unseen and destroy them.
Both countries also relied heavily on naval mines. An estimated 190,000 mines were placed in the North Sea during WWI in hopes that enemy ships would strike them.
The region was also the site of the Battle of Jutland, fought off Hanstholm in Northern Jutland. It became the largest naval clash in history. German and British fleets battled across the North Sea, more than 25 ships were sunk, and nearly 10,000 sailors died.
Those wartime losses helped leave the North Sea seabed scattered with wrecks, many of which remain there today.
More Than 10,000 Wrecks in Danish Waters
UC-30 represents only a small part of the problem.
In 2024, Andresen prepared a report for the Danish Environmental Protection Agency examining the environmental risks posed by historical wrecks in Danish waters. The work also considered possible ways to prevent or reduce the pollution they can cause.
"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 Environmen…