Just before 10:10 on a warm summer night in 1917, German soldiers loaded a new type of armament into their artillery and began bombarding enemy lines near Ypres in Belgium. The shells, each emblazoned with a bright yellow cross, made a strange sound as their contents partly vaporized and showered an oily liquid over the Allied trenches.

The fluid smelled like mustard plants, and at first it seemed to have little effect. But it soaked through the soldiers’ uniforms, and eventually it began burning the men’s skin and inflaming their eyes. Within an hour or so, blinded soldiers had to be led off the field toward the casualty clearing stations. Lying in cots, the injured men groaned as blisters formed on their genitals and under their arms; some could barely breathe.

The mysterious shells contained sulfur mustard, a liquid chemical-warfare agent commonly—and confusingly—known as mustard gas. The German attack at Ypres was the first to deploy sulfur mustard, but it was certainly not the last: Nearly 90,000 soldiers in all were killed in sulfur mustard attacks during the First World War. And although the Geneva Convention banned chemical weapons in 1925, armies continued manufacturing sulfur mustard and other similar armaments throughout the Second World War.

When peace finally arrived in 1945, the world’s military forces had a major problem on their hands: Scientists did not know how to destroy the massive arsenals of chemical weapons. In the end, Russia, the United Kingdom, and the United States largely opted for what seemed the safest and cheapest method of disposal at the time: Dumping chemical weapons directly into the ocean. Troops loaded entire ships with metric tons of chemical munitions—sometimes encased in bombs or artillery shells, sometimes poured into barrels or other containers. Then they shoved the containers overboard or scuttled the vessels at sea, leaving spotty or inaccurate records of the locations and amounts dumped.

Experts estimate that 1million metric tons of chemical weapons lie on the ocean floor—from Italy’s Bari harbor, where 230 sulfur mustard exposure cases have been reported since 1946, to the U.S.'s East Coast, where sulfur mustard bombs have shown up three times in the past 12 years in Delaware, likely brought in with loads of shellfish. “It’s a global problem. It’s not regional, and it’s not isolated,” says Terrance Long, chair of the International Dialogue on Underwater Munitions (IDUM), a Dutch foundation based in the Hague, Netherlands.

Today, scientists are looking for signs of environmental damage, as the bombs rust away on the seafloor and potentially leak their deadly payloads. And as the world’s fishing vessels trawl for deep-diving cod and corporations drill for oil and gas beneath the ocean floor and install wind turbines on the surface, the scientific quest to locate and deal with these chemical weapons has become a race against the clock.

On a rainy day in April, I hop a tram to the outskirts of Warsaw to meet Stanislaw Popiel, an analytical chemist at Poland’s Military University of Technology. An expert on the world’s submerged chemical weapons, the graying researcher takes more than an academic interest in sulfur mustard: He has seen the dangers of this century-old weapon close up.

I had hoped to visit Popiel in his Warsaw lab, but when I contacted him a day earlier by phone, he apologetically explained that it would take weeks to get the permissions necessary to visit his lab in a secure military complex. Instead, we meet in the lobby of a nearby officers’ club. The chemist, wearing a rumpled gray blazer, is easy to spot among the officers milling around in starched, drab green dress uniforms.

Leading me upstairs to an empty conference room, Popiel takes a seat and opens his laptop. As we chat, the soft-spoken researcher explains that he started working on Second World War sulfur mustard after a major incident nearly 20 years ago. In January 1997, a 95-metric-ton fishing vessel named WLA 206 was trawling off the Polish coast, when the crew found an odd object in their nets. It was a five- to seven-kilogram chunk of what looked like yellowish clay. The crew pulled it out, handled it, and set it aside as they processed their catch. When they returned to port, they tossed it in a dockside trash can.

The next day, crew members began experiencing agonizing symptoms. All sustained serious burns and four men were eventually hospitalized with red, burning skin and blisters. The doctors alerted the authorities, and investigators took samples from the contaminated boat to identify the substance and then traced the lump to the city dump. They shut down the area until military experts could chemically neutralize the object—a chunk of Second World War sulfur mustard, frozen solid by the low temperatures on the seafloor and preserved by the below-zero winter temperatures onshore.

A sample made its way to Popiel’s lab, and he began studying it to better understand the threat. Sulfur mustard’s properties, Popiel says, make it a fiendishly effective weapon. It’s a hydrophobic liquid, which means it’s hard to dissolve or wash off with water. At the same time, it’s lipophilic, or easily absorbed by the body’s fats. Symptoms can take hours or, in rare instances, days to appear, so victims may be contaminated and not even realize they have been affected; the full extent of the chemical burn might not be clear for 24 hours or more.

A chemist in Popiel’s lab discovered firsthand how painful such a burn could be, after a fume hood pulled vapors from a test tube full of the stuff up over his unprotected hand. The gas burned part of his index finger, and it took two months to heal—even with state-of-the-art medical care. The pain was so severe that the chemist sometimes couldn’t sleep more than a few hours at a time during the first month.

Popiel explains that the more he read about sulfur mustard after the WLA 206 incident, the more he began to question why it had survived so long on the ocean floor. At room temperature in the lab, sulfur mustard is a thick, syrupy liquid. But under controlled lab conditions, pure sulfur mustard breaks down into slightly less toxic compounds like hydrochloric acid and thiodiglycol. Bomb makers reported that sulfur mustard evaporated from the soil within a day or two during warm summer conditions.

But it seemed to remain strangely stable underwater, even after the metal casing of the bombs corroded. Why? To gather clues, Popiel and a small group of colleagues began testing the WLA 206 sample to identify as many of its chemical constituents as they could. The findings were very revealing. Military scientists had weaponized some stocks of sulfur mustard by adding arsenic oil and other chemicals. The additives made it stickier, more stable, and less likely to freeze on the battlefield. In addition, the team identified more than 50 different “degradation products” that formed when the chemical weapon agent interacted with seawater, sediments and metal from the bomb casings.

All this led to something that no one had predicted. On the seafloor, sulfur mustard coagulated into lumps and was shielded by a waterproof layer of chemical byproducts. These byproducts “form a type of skin,” says Popiel, and in deep water, where temperatures are low and where there are few strong currents to help break down the degradation products, this membrane can remain intact for decades or longer. Such preservation in the deep sea had one possible upside: The coating could keep weaponized sulfur mustard stable, preventing it from contaminating the environment all at once.

Some of the world’s militaries did dump their chemical weapons in deep water. After 1945, the U.S. military required that dump sites be at least 1,800 meters below the surface. But not all governments followed suit: The Soviet military, for example, unloaded an estimated 15,000 tonnes of chemical weapons in the Baltic Sea, where the deepest spot is just 459 meters down and the seafloor is less than 150 meters deep in most places—a recipe for disaster.

(Nearly a century has passed since the first use of sulfur mustard as a chemical weapon in the First World War, but these munitions remain a threat. This interactive map, created with data supplied by the James Martin Center for Nonproliferation Studies in Monterey, California, shows known locations where chemical weapons were dumped in the world’s oceans. Click on the map icons to view details about the sites; click on the slider icon on the top left to organize the content differently.)

On the day I arrive in the Polish resort town of Sopot, I take a short stroll along the seaside. Looking around, I find it hard to imagine that metric tons of rusting bombs packed with toxic chemicals lie less than 60 kilometers offshore. Restaurants on the town’s main drag proudly advertise fish and chips made with Baltic-caught cod on their menus. In the summer, tourists jam the white-sand beaches to splash in the Baltic’s gentle waves. Venders hawk jewelry made from amber that has washed ashore on local beaches.

I had taken the train from Warsaw to meet Jacek Beldowski, a geochemist at the Polish Academy of Science’s Institute of Oceanography in Sopot. From his cramped office on the second floor of this research center, Beldowski coordinates a team of several dozen scientists from around the Baltic and beyond, all working to figure out what tens of thousands of metric tons of chemical weapons might mean for the sea—and the people who depend on it.

Beldowski has a long ponytail and an earnest, if slightly distracted, manner. When I ask him if there’s anything to worry about, he sighs. With 4.7-million euro (U.S. $5.2-million) in funding, the project Beldowksi now leads is one of the most comprehensive attempts yet to evaluate the threat of underwater chemical munitions, and he’s spent the past seven years refereeing fractious scientists and activists from around the Baltic and beyond who argue over this very question.

On one side, he says, are environmental scientists who dismiss the risk altogether, saying that there’s no evidence the weapons are affecting fish populations in a meaningful way. On the other are advocates concerned that tens of thousands of uncharted bombs are on the verge of rusting out simultaneously. “We have the ‘time bomb and catastrophe’ approach versus the ‘unicorns and rainbows’ approach,” Beldowski says. “It’s really interesting at project meetings when you have the two sides fighting.”

To try to answer this big question, Beldowski’s collaborators first had to locate dump sites on the seafloor. They knew from archival research and other information that post-war dumping was concentrated in the Baltic’s three deepest spots—the Gotland Deep, Bornholm Deep, and Gdansk Deep. Beldowski calls up an image on his computer, created with side-scan sonar technology a few weeks earlier during a cruise on the institute’s three-masted research vessel. In shades of orange and black, the high-resolution image shows a two-square-kilometer patch of the Bornholm Deep, 200 kilometers from Sopot. Scattered across the image are nine anomalies that Beldowski identifies as individual bombs.

Running his cursor over the image, Beldowski points out long, parallel scratches on the seafloor. They’re telltale traces of bottom-dragging nets, evidence that trawlers have been fishing for cod in a known dump site although nautical charts warn them to stay away. “It’s not good to see so many trawl marks in an area where trawling is not advised,” Beldowski says. Worse still, many of the lines are near known bombs, so it’s very likely, he adds, that the trawlers uncovered them.

Once the researchers locate either bombs or scuttled ships with sonar, they maneuver a remotely operated submersible fitted with a camera and sampling gear to within 50 centimeters of the decaying bombs to collect seawater and sediment. Beldowski calls up a short video on his computer, taken from the remotely operated vehicle a few weeks earlier. It shows a ghostly black-and-white image of a wrecked tanker, resting about 100 meters below the surface.

Records suggested it was filled with conventional weapons when it was scuttled, but Beldowski says sediment samples taken from the ocean floor near the ship yielded traces of chemical agents. “We think it had a mixed cargo,” he says. In a lab down the hall from Beldowski’s office, samples from the ship are being analyzed using several different types of mass spectrometers. One of these machines is the size of a small refrigerator. It heats samples to 8,000 °C, cracking them into their most basic elements. It can pinpoint the presence of chemicals in parts per trillion.

Earlier research projects on Baltic water quality looked for traces of laboratory-grade sulfur mustard as well as one of the degradation products, thiodiglycol, and found next to nothing. “The conclusion was that there was no danger,” says Beldowski. “But that seemed strange—so many tonnes of chemicals and no trace?”

So Beldowski and his colleagues looked for something very different, based on Popiel’s research. They searched for the complex chemical cocktail that military scientists used to weaponize some stocks of sulfur mustard, as well as the new degradation products created by the munitions’ reaction with seawater. The team found sulfur mustard byproducts in the seafloor sediment and often in the water around dumped bombs and containers.

“In half of the samples,” says Beldowski, shaking his head, “we detected some degradation agents.” It wasn’t all sulfur mustard, either: In some samples, the degradation products came from other types of dumped chemical weapons, like nerve gas and lewisite.

Learning to detect these toxic substances is just part of the problem: Assessing the threat these chemicals pose to marine ecosystems and to humans is a more troubling issue. Although researchers have long gathered data on the dangers of toxins such as arsenic, the perils posed by weaponized sulfur mustard and its degradation products are unknown. “These compounds are weapons, so it’s not something you just give a grad student and tell them to run it,” says Hans Sanderson, an environmental chemist and toxicologist based at the Aarhus University in Denmark.

Sanderson thinks it would be irresponsible to hit the panic button until more is known about these munitions on the seafloor and their effects. “There are still lots of questions about the environmental impact,” the Danish researcher says. “It’s difficult to do risk assessment if you don’t know the toxicity, and these are unknown chemicals that nobody’s ever encountered or tested.”

Some scientists think that preliminary data on the effects of these chemicals on ecosystems might come from long-term studies of cod stocks. Cod is a commercially important species in the Baltic, so researchers from around the region have detailed records on these stocks and their health going back more than 30 years. And since cod are deep divers, they are more likely than many other Baltic fish to come in contact with sediment at the bottom of the sea—and with chemical munitions.

Thomas Lang, a fisheries ecologist at Germany’s Thünen Institute, is studying possible impacts of this contact. If cod caught near dump sites are more diseased than those pulled up from areas deemed “clean,” it could be a hint that the chemicals are harming the fish. “We use diseases as indicators of environmental stress,” Lang says. “Where fish have a higher disease load, we think the environmental stress is higher.”

Over the past five years, Lang has examined thousands of cod, looking at health indicators such as the mathematical relationship between their weight and length, and examining the fish for signs of illness and parasites. At the beginning of these studies, the cod caught from a major chemical weapons dump site seemed to have more parasites and diseases and were in poorer condition than those caught outside the dump area—a bad sign.

The latest data, however, paints a different picture. After 10 separate research cruises and 20,000 cod physicals, Lang’s study shows only tiny differences between fish caught in known dumping grounds and those taken from sites elsewhere in the Baltic. But Lang says that situation could change, if leaks of toxic substances increase due to corroding munitions. “Further monitoring of ecological effects is required,” he adds.

A small number of studies conducted elsewhere also raise doubts about the polluting effects of submerged chemical weapons. The Hawai'i Undersea Military Munitions Assessment (HUMMA), a project paid for by the US Department of Defense and run primarily by researchers from the University of Hawai'i at Manoa, is a case in point. Its scientists have been investigating a site near Pearl Harbor, where 16,000 sulfur mustard bombs were dumped in 1944.

Water samples taken by the HUMMA team confirmed the presence of sulfur mustard byproducts at the site, but time-lapse video shows that many marine species now use the bombs as an artificial reef. Sea stars and other organisms have shifted onto the piles of munitions, seemingly unaffected by the leaking chemicals. At this site, sulfur mustard “does not pose a risk to human health or to fauna living in direct contact with chemical munitions,” the researchers reported.

What is certain, however, is that the chemical weapons lying on the seafloor pose a serious threat to humans who come in direct contact with them. And as the world focuses more on the oceans as a source of energy and food, the danger presented by underwater munitions to unsuspecting workers and fishing crews is growing. “When you invest more in the offshore economy, each day the risk of finding chemical munitions increases,” Beldowski says.

Indeed, some major industrial projects in the Baltic, such as the Nord Stream gas pipeline from Germany to Russia, are now planning their routes in order to avoid disturbing chemical weapon dumps. And trawler activity on the ocean floor continues to uncover chemical munitions. In 2016 alone, Danish authorities have responded to four contaminated boats.

Yet there are some options for cleaning up the mess. Terrance Long, at the IDUM, says encasing the corroding munitions in situ in concrete is one possible option. But it would be expensive and time-consuming. Beldowski says it might just be easier for now to place fishing bans and stepped-up monitoring around known dump sites—the nautical equivalent of “Do Not Enter” signs.

As I pack away my notebook and get ready to head back to the train station in Sopot, Beldowski still looks worried. He thinks that scientists need to remain vigilant and gather more data on what is happening in the seas around those dump sites. It took decades, he says, for scientists across many disciplines to understand how common chemicals such as arsenic and mercury build up in the world’s seas and soils, and poison both wildlife and people. The world’s seas are vast, and the data set on chemical weapons—so far—is tiny.

“Global collaboration made the study of other contaminants meaningful,” Beldowski says. “With chemical munitions, we’re in the same place marine pollution science was in the 1950s. We can’t see all the implications or follow all the paths yet.”

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