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 1 million 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.

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.

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.”

Hans Sanderson, an environmental chemist and toxicologist based at the Aarhus University in Denmark, 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.”

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.

Beldowski 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.”