What would you do with 65,000 tonnes of dangerous chemical weapons? Given what we know now about marine pollution, your answer — I hope — would not be to dump the lot at the bottom of the ocean. 

But that's exactly what the victorious allies did after the end of World War II. Following an agreement reached at the Potsdam Conference of 1945, Soviet and British forces disposed of a massive stockpile of captured chemical weapons by sinking them in the Baltic Sea. Even more disturbingly, according to a recent article in The Economist, not all of the dumping occurred where it was supposed to: 

Although the vast majority of the munitions were thrown into the Bornholm and Gotland deeps, Jacek Beldowski, from the Polish Institute of Oceanography, said the Soviets often threw everything overboard “as soon as they were out of sight of land.” This means there could be tonnes of chemical weapons lying in unknown locations, close to land and in fishing zones. 
Indeed researchers, including Beldowski, have found an increase in sick and mutated fish around dumping zones and traces of mustard gas have been detected just a few hundred meters off the Polish coast, an area nowhere near an official dumping ground. 

Hindsight is, of course, a wonderful thing.

As Mike noted in discussing this story over at TreeHugger, our methods for disposing of chemical weapons have certainly improved. We may look back at past generations and ask: "What were they thinking?" Yet before pointing the finger, we'd do well to look around at our own activities today. We continue to pollute land, air and sea in countless other ways without fully understanding the consequences of what we are doing.  

What don't we know, and how might it come back to bite us?

Oh, and just in case you were thinking you don't live near the Baltic, check out where else chemical weapons have been dumped. (Hint: they're everywhere)

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