ON AUGUST 28, 1914, BRITISH BATTLECRUISERS brought a trail of destruction to the North Sea when they ambushed and sank SMS Ariadne, the Imperial German Navy’s 344-foot-long colonial light cruiser, near a stretch of sea just off the coast of Germany. The cruiser sank while carrying massive SK L/40 guns and torpedo missiles, a host of other weaponry, and 712 sailors. This came to be known as the Battle of Heligoland Bight, the first Anglo-German naval clash of World War I.
I had been considering the skeleton of this great wreck as I climbed aboard the 180-foot-long research vessel Heincke. It was February 2023, and I had arrived in Bremerhaven, Germany, a sprawling North Sea port, to join a mission hellbent on unraveling the hidden dangers lurking beneath these waters. Heincke, a workhorse fitted with two cranes, an A-Boom, multiple winches, and four state-of-the-art laboratories designed for multidisciplinary marine research, was well-equipped for the task. Below deck, the ship’s laboratories offered spaces for analyzing biological, geological, and hydrographic samples. She set off carrying a group that included four researchers, a six-man deck crew, one cook, the captain, and myself. Over the next few days, we’d be investigating the vast underwater graveyard of unexploded ordnance (UXO) scattered across the North Sea, a perilous legacy of two World Wars that still threatens marine ecosystems—and human life. These munitions, from bombs and torpedoes to chemical agents, lie hidden in the seabed, their deteriorating casings leaking toxic remnants into the sea.
Throughout the course of both World Wars, an estimated 1.3 million metric tons of conventional munitions—think bombs, torpedoes, shells, rockets, small arms, sea and landmines—would be scattered along Germany’s coastal waters, largely in the North Sea. Around 30,000 tons of chemical warfare agents (CWA)—the chlorine, phosgene, and mustard gas that choked World War I combatants to death or burned their skin—pollute the neighboring Baltic Sea, which also conceals about 300,000 tons of conventional munitions and weapons. Much of this detritus is the result of a violent forced disarmament the Allied powers imposed on Germany after World War II.
These waters likely hide even higher volumes of weapons, but much of the submerged stockpile remains undocumented. As part of the disarmament, the Allied powers hastily organized larger ammunition dump sites throughout the North Sea. These were mapped and charted in the 1950s and 1960s, allowing fishing boats to avoid them, but countless smaller or unofficial dumpsites remain unmarked. Ill-prepared ships and German fishermen assigned with the task of dumping UXO into the sea often lacked proper guidance or equipment. The objective was to get rid of the munition as soon as possible. This mentality led to haphazard and undocumented scatterings, with munitions dumped in random locations across the North Sea, far beyond the designated sites. Today, fishermen face significant risks when their nets inadvertently ensnare UXO, sometimes with grave consequences, as evident in fatal incidents where explosives were trawled aboard ships. Divers who maintain oil rigs or lay undersea cables also encounter these hazards, often working dangerously close to munitions lying dormant on the seabed. Between 1945 and 2008, more than 115 people died from encounters with UXO in the German North Sea.
But the ocean works relentlessly. In the decades since their disposal, some metal components of weaponry and ammo casings have decayed with the aid of salt water and bacteria. The speed and severity of this process is influenced by environmental factors like salinity, depth, and temperature, so the impacts vary throughout the North Sea. This makes pollution from the munitions tricky to study for the teams currently out on the waters, and scientists still haven’t fully grasped exactly how old munitions fare in marine environments. They suspect that the degradation products—chemicals like dinitrotoluenes (DNTs) and trinitrobenzene, which form as explosives break down—persist even longer and have wider-reaching effects on surrounding ecosystems than the weapons themselves, meaning that while these unexploded ordnances are quite literally ticking time bombs in their own right, the real danger lies in the slow, invisible process of chemicals leaching into the local marine environment.
“You could eat [these fish] without having an enhanced risk of getting cancer. However, if you, as a fish, swim in these contaminated waters 24/7, then the risk of getting diseases in organs like the liver obviously increases,” said Matthias Brenner, Ph.D, a marine biologist at the Alfred Wegener Institute for Polar and Marine Research (AWI) in Germany, who was aboard Heincke for the journey. Consumers living off the North Sea and other munitions-polluted waters may face risks as seafood continues to absorb these toxins.
To trace the effects of unexploded ordnance on both people and wildlife, scientists like Brenner are examining the North Sea with geological surveys, microbial analyses, and wildlife and water sampling. And crucial evidence has already surfaced. For example, scientists have found that the muscle tissues of fish living near the wrecks contained traces of trinitrotoluene (TNT), an explosive substance used in artillery shells, grenades, and airborne bombs. Studies show TNT and its metabolites accumulating in fish near munition sites in the North Sea, with concentrations reaching up to 4 nanograms per gram (ng/g) in flatfish muscle tissues.
The first night onboard Heincke was peaceful, all things considered. We’d already anchored at Heligoland, Germany’s only open-sea island and a spectator to the brutal battle that sank SMS Ariadne over a century ago. Meanwhile, Europe’s most turbulent body of water stretched endlessly around us, tempestuous and unyielding. Shallow and full of varying tidal patterns, it boasts a rich marine life, but winter cruising is no walk in the park—it’s more like nature’s bootcamp; even while anchored, the ship rolled and pitched among the waves, and I battled lingering seasickness. All passengers met for dinner at 5:30 p.m. sharp, and we never dined on the day’s catches—these fish were strictly for study.
Avoiding the local seafood was partly a decision based on Brenner’s prior work. From 2011 to 2014, he collaborated on the Baltic Sea-based Chemical Munitions Search & Assessment (CHEMSEA) project, which revealed that the scale of pollution there may be more widespread than scientists initially expected and that toxins may be traveling from wreck sites via water currents. The methods used for CHEMSEA laid a helpful foundation for similar work in the North Sea, which is currently being spearheaded by North Sea Wrecks (NSW), a European Union-funded project. Between 2018 and 2023, NSW investigated 21 wrecks located between the Belgian Coast and Skagerrak, a strait separating Norway and Denmark. The findings confirmed that munitions and their toxic byproducts are leaching into the surrounding marine environment, with pollutants like TNT and its degradation products detected in sediment and marine organisms near the wrecks. These results underscored the urgent need for remediation efforts.
After hours of sitting with my own mal de mer, Heincke at last approached the vicinity of SMS Ariadne, one of several wrecks scattered across Heligoland Bight. That day, the divers picked up from Heligoland would attempt to retrieve a lost lander near the resting site of SMS Mainz, another light cruiser of the German Imperial Navy sunk during the Battle of Heligoland. This lander, which is used to carry research samples and tools between the surface and seafloor, had been lost two years earlier, along with valuable biological samples, including marine organisms and microorganisms that had settled on or near the frame. These can provide eye-opening insights into an ecosystem’s health because they sit at the base of the food chain, recycle nutrients and organic matter, and quickly respond to environmental shifts. Retrieving the lander could offer rare insights into the health of the ecosystem surrounding the wreck, helping researchers better understand how pollution from UXO impacts marine life over time.
Scientific dives in Germany require three people: a dive mission leader, a mission diver, and a safety diver. Divers enter the water alone, equipped with a communication device attached to their face mask, which transmits voice signals to the surface. Depending on the diver’s depth, they use single or double cylinders filled with compressed air or gas mixtures like nitrox or trimix, carrying up to nearly 90 pounds on their backs. “In the water, however, all of this weight diminishes, and you achieve a feeling of zero gravity if you enter the state we call neutral buoyancy,” said Henning Hoffman, a dive team member and marine zoologist at AWI. Unfortunately, the dive to recover the lost lander proved fruitless due to poor visibility. “The North Sea is unforgiving,” Hoffman remarked as the team regrouped.
By the following morning, the weather hadn’t improved enough for diving. But the show went on with remote sampling. The researchers first deployed an onboard multibeam, a system that can pinpoint wrecks with acoustic waves. Next, they sent down a rosette sampler, a device attached to a wire that opens large metal bottles to collect water samples. It stopped directly above the wreck of SMS Mainz, near the site where the lost lander was believed to be located, as well as 16 and 32 feet above it, intervals chosen to capture water samples from different depths, allowing researchers to analyze how contaminants disperse vertically through the water column. Another device equipped with various sensors measured the water pressure, electrical conductivity, and temperature at various parts of the ship. A lawnmower-like device collected a sample of sediment from the seabed around various parts of the ship. Finally, the team caught some flatfish using standard bottom trawl gear near the wreck, aiming to analyze the levels of pollutants present in marine organisms.
Back on Heincke, in one of its labs, AWI scientists dissected the still-living fish and removed the intestines. They quickly placed samples, including specimens of bile, liver, fillet, and kidneys, in liquid nitrogen to preserve them for later gene and protein analysis. Among the flatfish caught by the Heincke researchers were common dabs (Limanda limanda), which are found in the sandy and muddy seabeds of the North Sea and the Baltic. This species, often a staple in European coastal cuisines, is prized for its delicate texture and mild flavor and rarely grows longer than 20 centimeters. The team found the common dabs’ tissue dotted with small nodules of a sickly pinkish color. These are indicative of cancer in the fish, Brenner said.
Elevated cancer rates in flatfish around munitions dump sites were first hypothesized by biologist Thomas Lang and his colleagues two decades ago, based on his sampling of thousands of flatfish in the Baltic. Jörn Scharsack, Ph.D., a fish pathologist at the Thünen Institute of Fisheries Ecology in Germany, is currently studying the health of flatfish in the North Sea. While Scharsack expects his ongoing research to corroborate Lang’s findings in the Baltic, he emphasizes the need for larger sample sizes to draw definitive conclusions.
One challenge with such research, he explains, lies in comparing populations with varying age distributions. “Older fish are more likely to develop cancer, and the lack of fishing pressure at dumpsites means these areas are dominated by older populations,” Scharsack said. This complicates direct comparisons with younger control groups, but he thinks the evidence of disease near munitions sites remains compelling. To overcome this hurdle, the Heincke scientists usually aim to catch 50 specimens that are about 25 centimeters in length. Fish of this size are four to five years old, on average, Brenner said.
Cancer is not the only risk. Following the expedition, toxicology labs at the University of Kiel in northern Germany analyzed the water, sediment, and tissue samples gathered by the Heincke researchers, looking for evidence of explosives, and additional tissues were sent to AWI labs in Bremerhaven, Germany, for microscopic study. The results were troubling: the fish from SMS Ariadne’s periphery showed clear signs of liver disease linked to TNT contamination. Simply put, they observed that the more TNT present, the higher the number of diseased fish in a given population. They later compared these findings to samples collected at a reference site in a North Sea nature reserve off Germany’s coast, which showed significantly lower levels of dissolved explosives.
Despite some concerning data, it’s hard to directly link specific health impacts to particular pollutants, especially over many decades and in the context of people’s broader diets. Compounding this issue is the deluge of pollutants from 20th-century industrialization, like microplastics and pesticides. The European Food Safety Authority sets benchmark levels for other pollutants, like lead and arsenic, well below hazardous limits to ensure consumer safety. But no expert body has established a universally accepted “safe” level for substances like TNT in seafood. Regardless, “the current results are highly alarming,” said Ute Marx, Ph.D., a biological technical assistant at AWI. In her words, we are facing an “underwater ticking bomb.”
On the final day of the expedition, Heincke returned the divers to Heligoland before conducting one last round of fishing over Ariadne. After the ritualistic 5:30 p.m. dinner, I bade farewell to the AWI team, the Heincke crew, and our captain, and made my way to Bremerhaven’s train station. The image of the common dabs’ pink-spotted tissue lingered in my mind as I stood shivering on the platform.
But no matter how far from the North Sea one flies, the problem of underwater munitions extends far beyond it, spanning oceans and continents. A global map of munitions dump sites from the James Martin Center for Nonproliferation Studies reveals clusters stretching from the North Pacific Ocean to the eastern shores of Australia. Estimates suggest that there are between 150 to 300 sites globally with dumped chemical weapons like mustard gas, phosgene, and sarin. The highest volumes are found in areas with histories of intense conflict like the Baltic and North seas, the eastern Atlantic Ocean, and off of the West Coast of the U.S. Unregulated hazardous waste dumping persisted on the world’s waters until it was banned globally in 1975 by the London Convention.
Beyond studying its impacts, experts are working to shore up much of this underwater arsenal. Groups from around the world have worked to facilitate munitions extraction efforts, including the Dutch-Canadian organization, International Dialogue on Underwater Munitions, that was founded in 2004. In UXO hotbed Germany, alone, the state’s Explosive Ordnance Disposal (EOD) team in Schleswig-Holstein has tackled this hazardous legacy head-on since 2015. Altogether, the team has placed more than 70 tons of ammunition in a designated ocean dumping ground and disposed of another 33 tons on land, said Oliver Kinast, EOD director. They have also disarmed more than 80 ground mines found in the ocean, though they can’t be brought onshore because of risks of explosion.
But many of the undersea sites for old munitions have a major flaw. Due to the urgency and lack of foresight at the time, the dump sites organized by Allied forces in the years following World War II were nothing more than open stretches of seabed, often near shipwrecks or scattered along transport routes. These sites lacked containment systems to prevent hazardous substances from leaching into the environment. Compare those to modern, land-based hazardous waste disposal sites, which include features like protective linings and leachate collection systems to prevent contaminants from infiltrating the surrounding soil and groundwater. These are also subject to strict regulations and monitoring, which are lacking for munition dump sites in water. Such a lack of important safeguards underwater has allowed toxins to spread through marine ecosystems unchecked, creating a slow-burning environmental crisis.
But shiny new technologies could breathe new life into munitions remediation. “Cleaning up UXO will be a booming business,” said Dieter Guldin, chief operations officer at SeaTerra, a German company specializing in UXO detection and clearance on both land and sea. The goal is to securely eliminate the weapons in the waters without bringing them ashore, reducing the risks to populated areas. Currently, the German government is working with researchers and businesses on a robot-powered system called RoBEMM to grab, disassemble, and safely dispose of UXO in the North and Baltic seas. The precise disposal techniques are still under development, and RoBEMM participants said they can’t provide specific details at the moment. And even if they pan out, high-tech remediation schemes won’t be enough to completely rid the North and Baltic seas of these looming dangers. “At the current speed, even if all companies available on the market work on remediation, we would need up to 900 years to get the job done,” Guldin said.
The remediation effort will require all hands on deck, including research initiatives like NSW. In July 2023, the NSW project metamorphosed into REMARCO (Remediation, Management, Monitoring, and Cooperation) to continue addressing North Sea UXO. REMARCO has received $2.9 million in funding from the EU, which has recently provided billions of dollars to similar work from other institutions throughout Europe. REMARCO will continue scouring the sea until June 2027, and the team is experimenting with ways to continuously monitor North Sea munitions and remediate both wrecks and dumping sites, Brenner said. A remotely operated crawler equipped with a manipulator arm has already successfully taken a “dive” underwater, collected sediment and water samples, and returned them to the boat for analysis. This handy tool has helped researchers study how bacteria in sediments react to TNT, potentially providing indicators—like changes in microbial activity or population dynamics—that signal contamination hot spots. The REMARCO team has also been sampling marine organisms like crabs, which haven’t been extensively studied in this context, to trace the spread of pollutants through ecosystems.
While no one is entirely sure when and how these risks will travel through marine ecosystems and onto people’s plates, researchers and regulators have agreed that UXO cleanup is a timely issue. “No matter how deep munitions and wrecks are, sooner or later they all pose a serious threat to all life in the sea and, thus, also to us,” said Ute Marx of AWI.
Stav Dimitropoulos’s science writing has appeared online or in print for the BBC, Discover, Scientific American, Nature, Science, Runner’s World, The Daily Beast and others. Stav disrupted an athletic and academic career to become a journalist and get to know the world.
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