Millions of tons of bombs, shells, mines, and chemical weapons sit on the ocean floor, mostly dumped there on purpose after World War I, World War II, and the Cold War. At the time, sinking unwanted munitions at sea seemed like the easiest and cheapest disposal method. Governments around the world loaded surplus and captured weapons onto old ships, barges, and rail cars, then pushed them overboard in designated dump zones. Today, those corroding weapons are leaking toxic chemicals into marine ecosystems and creating hazards for fishing boats, divers, and offshore construction crews.
How the Bombs Got There
The bulk of ocean munitions trace back to the years immediately following the two world wars. After each conflict, Allied and Axis nations held enormous stockpiles of conventional bombs, artillery shells, naval mines, torpedoes, and chemical weapons they no longer wanted. Destroying munitions on land was expensive, slow, and politically unpopular near populated areas. Ocean dumping was fast: load a ship, sail it to a designated zone, and push everything overboard. In some cases, entire decommissioned warships loaded with munitions were simply scuttled.
Chemical weapons received the same treatment. After World War II, Allied forces seized tens of thousands of tons of German chemical munitions, including mustard gas and arsenic-based agents like Clark I, Clark II, and Adamsite. Rather than dismantle them, military authorities dumped them at sea in bulk. The Soviet Union, the United States, the United Kingdom, and other nations all participated in chemical weapons dumping through the 1940s, 1950s, and into the 1960s.
Not all underwater munitions were deliberately dumped. Some are unexploded ordnance (UXO) from naval battles, aerial bombing runs, and training exercises. Mines laid during wartime were never fully cleared. Planes crashed carrying live bombs. Ships sank with full magazines. The result is a patchwork of both intentional dump sites and scattered individual munitions across coastal waters worldwide.
Where the Largest Dumps Are
The highest concentrations sit in European waters. More than 1.5 million tons of ammunition lie on the floor of the North Sea and Baltic Sea combined, according to Germany’s GEOMAR Helmholtz Centre for Ocean Research. The German Bight alone, a section of the southeastern North Sea, holds an estimated 1.3 million tons of unexploded ordnance. In the Baltic Sea, munitions are clearly visible on the seabed and have been documented and mapped using underwater robots.
U.S. coastal waters hold their own share. The Bureau of Ocean Energy Management notes that UXO are found offshore in most U.S. coastal areas. Major dump sites exist off the coasts of New Jersey, South Carolina, California, and Hawaii, among others. The U.S. military used dozens of designated ocean disposal zones from the 1940s through the early 1970s. Japan, Australia, and several other Pacific nations also have significant dump sites from World War II.
What’s Actually Down There
The munitions fall into two broad categories: conventional explosives and chemical weapons. Conventional munitions include aerial bombs, artillery shells, naval mines, torpedoes, grenades, and small arms ammunition. Many still contain live explosive fillers like TNT, which remains chemically stable for decades underwater. That stability is part of the problem: the explosive material doesn’t break down, even as the metal casing around it does.
Chemical munitions contain warfare agents such as sulfur mustard (mustard gas) and arsenic-based compounds. Research in the Baltic Sea has confirmed that these agents leak from corroded casings into surrounding sediments. Some of the breakdown products remain toxic even after chemical degradation in the marine environment. Sulfur mustard, for instance, can form a thick, tar-like crust on the outside of shells that persists for decades and still causes chemical burns on contact.
Why the Casings Are Failing Now
Saltwater is highly corrosive to steel. After 70 to 80 years of submersion, many munition casings have thinned, cracked, or collapsed entirely. This progressive corrosion is the primary mechanism by which toxic chemicals enter the water. Once a casing breaches, seawater dissolves the explosive filler or chemical agent inside, carrying contaminants into surrounding sediment and the water column.
TNT leaking from corroded conventional munitions is one of the best-studied examples. In mussels, TNT is rapidly converted into amino-dinitrotoluenes, a group of breakdown products that are actually taken up by living tissue more efficiently than TNT itself. Studies on salmon exposed to TNT-contaminated water found that one breakdown product accumulated in fish tissue at concentrations roughly 400 times greater than the surrounding water concentration. Even very low levels of TNT in water caused increased death rates in young fish, with lethal effects observed at concentrations as low as 0.4 milligrams per liter. Researchers have noted that developmental problems in fish may occur at concentrations even lower than that.
Risks to Marine Life and People
The ecological concern is twofold. First, toxic compounds leaking from munitions contaminate sediments and water, harming organisms that live on or near the seafloor. Filter feeders like mussels are particularly vulnerable because they process large volumes of water and accumulate contaminants in their tissue. Second, the breakdown products of TNT and chemical agents can move up the food chain through bioaccumulation, potentially reaching fish species consumed by humans.
For people, the most immediate danger is physical. Fishermen in the Baltic and North Seas regularly pull munitions up in their nets. Trawlers have brought up mustard gas shells that caused severe chemical burns to crew members. Offshore wind farm construction is another growing concern. Installing foundations and laying cables on the seabed requires disturbing the ocean floor, and in areas with dump sites, that means working around live explosives. The U.S. Bureau of Ocean Energy Management warns that detonation of UXO can cause shock waves strong enough to damage the seafloor, destroy boats or structures, and injure or kill both humans and marine animals. Even controlled detonation procedures are dangerous to divers.
Removing or relocating underwater munitions is not commonly done as a routine solution. It carries high risk, high cost, and can delay construction projects significantly.
Why Dumping Was Eventually Banned
International law caught up with the practice in the 1970s. The London Convention of 1972, one of the first global treaties aimed at protecting the marine environment from human activities, entered into force in 1975. It created a “blacklist” of materials that could no longer be dumped at sea. A stricter update, the London Protocol, was agreed in 1996 and took effect in 2006. Under the Protocol, all ocean dumping is prohibited unless the material falls on a narrow “reverse list” of possibly acceptable wastes. Incineration of waste at sea and the export of waste for ocean dumping are also banned.
These laws stopped new dumping but did nothing to address what was already on the seafloor. No international framework currently requires countries to clean up legacy dump sites, and the sheer scale of the problem makes comprehensive removal unrealistic with current technology and budgets.
Cleanup Efforts and Technology
Several large-scale research initiatives, particularly in Europe, are working to develop practical solutions. Germany’s Fraunhofer Society coordinates multiple projects targeting different parts of the problem. Some focus on detection and mapping, using AI-powered autonomous underwater vehicles to locate and identify munitions on the seabed. Others are developing automated recovery systems for fragile, corroded objects that are too dangerous to handle manually. One project is building a mobile demonstrator for disposing of large-caliber munitions in coastal areas, aiming to turn what has been a case-by-case effort into a repeatable standard procedure.
Other initiatives combine sensor technologies from aviation, space, and security industries to create comprehensive awareness of what lies above and below the water surface in dump zones. Passive sensors help identify munition types without physical contact, and teleoperated (remotely controlled) systems allow recovery without putting divers at risk. Predicting how deeply munitions have buried themselves in sediment and how far corrosion has progressed are active areas of research, since both factors determine how dangerous a given object is and what method can safely be used to deal with it.
None of these technologies have yet reached the point of large-scale commercial deployment. The projects are producing promising results in real-world testing, but cleaning up decades of ocean dumping across hundreds of sites worldwide will take sustained funding and international cooperation on a scale that doesn’t yet exist.