A Poisonous Catch

Off the hilly resort island of Bornholm, a tiny speck of Denmark that rises from the Baltic Sea between Sweden and Poland, the crew of the Danish trawler Soraya was catching cod one March morning. As the men hauled their fish on board, Theis Branick, 24, went beneath the net to open it. When the fish spilled out onto the deck, he found the net had also caught something else — a large, yellow-brown lump of solidified mustard gas from World War II.

"It was a huge piece, weighing about 15 kg, and with no traces of the...