Beneath the seas off Germany's northern coast, vast quantities of bombs and other weapons are slowly rusting away.

They are part of an estimated 1.6 million metric tons of conventional munitions in the North Sea and Baltic Sea, a legacy of two world wars and the mass disposal of weapons after 1945.

How old weapons pose a new threat

For decades, the material was largely left alone unless it posed an explosive danger. But as metal casings corrode, a different problem is emerging.

Some weapons are remnants of fighting: mines, unexploded bombs and ammunition lost at sea. But huge quantities were deliberately dumped after Nazi Germany surrendered in 1945, as the Allies disposed of the country's vast arsenal.

What was once regarded as an explosive hazard has now become an environmental one.

"It's clear that due to corrosion of these items, compounds enter the marine ecosystem and enter marine life," Kiel University toxicologist Edmund Maser told DW. "It turned out that these compounds are toxic and carcinogenic, both for human life and also for marine life," he said.

Among the most toxic compounds in the old weapons is trinitrotoluene, or TNT, a military explosive. Jörn Scharsack of Germany's Thünen Institute of Fisheries Ecology looked for evidence of it in dab fish.

"We find TNT in the fish, or let's say, metabolization, degradation products of TNT," he said.

The quantities are tiny — measured in nanograms — and not every fish contains detectable levels. But researchers repeatedly find the compounds around old munitions hotspots. "It's low. But it's there."

How toxic munitions are affecting marine life

So far, the contamination does not appear to be making fish acutely sick.

"The fish look healthy when we take them out. They do not have more diseases in terms of viral, bacterial, parasite diseases," Scharsack told DW. There is, however, one recurring abnormal finding.

"What we see is tumors, so tumors in the liver. That pops up again and again."

Maser's group has also found munitions compounds in fish, as well as mussels.

Together with the Alfred Wegener Institute (AWI) in Bremerhaven, his team found liver abnormalities in more than 50% of fish living around some munitions dumping grounds and wrecks, compared with normal rates of just 1–2%.

Mussels, meanwhile, showed biochemical signs of oxidative stress. As TNT is metabolized, highly reactive forms of oxygen can damage proteins, lipids and even DNA. "This can be a step towards cancer," said Maser.

And the contamination does not stop with fish and mussels.

Maser's team has also detected TNT-related compounds in tissue samples from two stranded whales, showing that the chemicals are reaching animals at the top of the marine food web.

"That means that these compounds have arrived in top predators," he said.

Are Germany's sea munitions dangerous to humans?

That raises the obvious question: If TNT-related compounds are reaching top predators, are people who eat seafood at risk?

For now, both scientists say the answer is no.

Whales face more constant exposure through prey and surrounding water, while humans are exposed only occasionally through seafood.

"One would need to eat lots of fish to really reach a level where it's getting dangerous," said Scharsack. "I'm still going swimming. I'm still eating fish."

The concern is what happens as the munitions continue to deteriorate.

"I would say it's likely to get worse if we look at the ongoing corrosion of the munitions," Scharsack said.

"The shells are corroding. More and more of the toxic substances are lying open, exposed to the water."

The problem appears worse in the Baltic than the North Sea because weaker currents and water exchange allow leaking pollutants to linger rather than disperse.

What mussels reveal about TNT contamination

Maser's research suggests this may already be happening. His team examined archived mussels dating back to 1985 and found TNT-related contamination increasing steadily from around 2000 through 2022.

"It's low, but it's increasing," he said. "And we expect that over the coming decades, maybe two or three or four decades, we will have concentrations that could maybe harm the human seafood consumer."

Marine life is already under pressure from what Maser described as a "whole cocktail" of human-made pollutants, including microplastics, pesticides and pharmaceutical compounds. Munitions add another contaminant to that mix.

Unlike many of those pollutants, which are already dispersed through the oceans, bombs and mines remain physical objects that can be salvaged and disposed of.

And the window will not stay open forever.

Germany has long removed munitions posing an immediate danger, but only recently began pilot projects for systematic clearance to prevent environmental harm.

"The topic has been ignored for decades. Nobody wanted to talk about it," Scharsack said.

What are the challenges in clearing munitions?

In a study published in July, GEOMAR researcher Torsten Frey and colleagues identified 484 known or potential munitions piles in two areas of the Baltic Sea. But of the estimated 1.6 million tons of munitions in German waters, around 1.5 million tons have yet to be mapped, Frey told DW.

Recovering the munitions from known locations is no longer the main obstacle, Frey said, with specialist companies able to clear large quantities relatively quickly. "The bottleneck is the disposal and destruction of the recovered munitions."

Germany is developing an offshore treatment platform to help increase disposal capacity. "This will be a marathon, not a sprint," said Frey.

Even clearance will leave small pieces of explosive behind, some no larger than "a sugar cube." Maser is investigating whether marine bacteria and fungi could eventually break down these residues into CO2 and water.

The work is still at the laboratory stage and Maser stresses that it cannot replace physical clearance.

"We need more of these platforms," he said. "We need to start as early as possible."

"If we wait maybe 40 years, then maybe all these metal cases are corroded and we have then no chance to get these munition compounds out of the water," said Maser. "This is why we should act now."

Edited by: Rosalie Delaney