The partly enclosed Baltic Sea is known as one of the most polluted seas in the world.

BBC: Sweden wants explanation on Baltic nuclear 'dumping'

The sea is enclosed and a discharge of toxic waste might cause devastating harm to the wildlife.

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Nan Madol comprises a set of almost 100 stone and coral fill platforms or artificial islets separated by narrow channels and enclosed by an outer sea wall.

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The Baltic Sea is semi-enclosed, so it takes a long time to flush out toxins.

BBC: Sweden wants explanation on Baltic nuclear 'dumping'