Baltic Sea trawlers sometimes dredge up a disturbing memory along with their catch: corroding chemical weapons from World War II.
Each time, a question is also brought to the surface: Is it safer to leave the shells and bombs alone or to try some remedial action to guard against a possible catastrophe?
Tons of captured Nazi chemical munitions – including some containing mustard gas blister agents and the infamous Zyklon B used in concentration camps – rest on the Baltic seabed. Estimates on the amount dumped by the Allies range from at least 13,000 tons to more than 200,000 tons.
Many experts believe the bulk of the chemical agents were quickly neutralized by sea water, and any remaining threat is gradually dissipating or buried safely under sediment.
But another camp, led by Russian scientists, is urging a more active approach. They worry the risks have been underrated and insist more monitoring is needed to determine whether the weapons should be recovered or perhaps sealed in watertight tombs.
The debate – the subject of many environmental studies and academic papers – has gained added prominence with the attention to chemical weapons during the war in Iraq. The European Union will also be facing more Baltic issues after its borders move eastward next year.
In early June, a trip around the Baltic Sea by environmentalists and religious leaders served as an open forum on the old chemical dump sites.
"The most dangerous thing is that we are not paying enough attention to the weapons and how they are affecting the surrounding ecosystem,"said Vadim Paka, a researcher at the Institute of Oceanography in Kaliningrad, a Russian enclave on the Baltic. "Forgetting about these weapons could be a tragic mistake."
But the Helsinki Commission, a regional environmental agency, sees limited concern.
"There’s been no catastrophe for more than 50 years, so that must mean something," said Juha-Markku Leppanen, who leads the group’s monitoring of the chemical weapons dump sites. "The consensus has been that it’s better to leave them alone than try to pick them up."
Some weapons were sent to the bottom on dozens of ships scuttled by the Allies. Other shells and containers were simply heaved overboard. At the time, it was considered the most expedient option to dispose of the dangerous arsenal, which included nerve agents such as tabun and stockpiles of mustard gas dating back to World War I.
Chemical weapons have been dropped into the sea all over the world, but the Baltic poses special challenges because of its heavily populated shores and its role as a shipping outlet to the North Sea and Atlantic.
The international Chemical Weapons Convention only requires cleanup for munitions dumped in the sea after 1985.
The sunken Nazi weapons are concentrated in the North Sea northwest of Denmark and in the Baltic near the Danish island of Bornholm and south of the Swedish island of Gotland, experts said. But smaller pockets are scattered through the shallow Baltic, whose average depth is just 171 feet and often within the reach of fishing nets.
More than 50 artillery shells or bombs holding 3,300 pounds of active gas have been snagged by nets since 1995, according to Helsinki Commission studies. In 1985, seven fishers were hospitalized in Copenhagen, Denmark, for exposure to an old mustard gas shell that leaked its content after it was hauled aboard their boat.
No serious contamination incidents have been reported since then, but fishing crews working near the dump areas often carry protective masks and gloves.
The main dump sites are well marked on maps. What’s less clear is whether the munitions are best left where they are. Parliament members and environmental groups in the countries surrounding the Baltic have asked for more studies on possible dangers.
"The studies say they don’t seem to pose a major threat to environment or humans," said John Hart, a researcher on chemical and biological weapons at the Stockholm International Peace Research Institute. "But that doesn’t mean they should be forgotten either."
Some chemical agents, such as tabun and Zyklon B, quickly dissolve in water and lose their potency, experts said. But Hart said some forms of mustard gas become viscous and break down very slowly in the chilly Baltic water and can be covered by a crusty shell that could crack open if hauled to the surface.
Hart also said more studies are needed to discover if arsenic compounds left over from chemical decomposition are entering the food chain through fish and other marine life.
David Santillo, a senior scientist with Greenpeace, said there needs to be a more unified effort to determine the weapons’ condition. "As long as they stay down there, they’re going to be a liability," he said.
Paka, the Russian researcher, also worries that sea currents could be shifting chemical shells and that corrosion could lead to more ruptures.
"We are not saying a catastrophe is waiting," he said. "What we do want is more monitoring. We can’t just rest on the reports we have. We cannot forget what we are dealing with. These weapons were designed to kill."