There's danger lurking（ 潜伏） in the Baltic（a波罗的海的） Sea, says Benedict Hutch. He's a marine（海的） researcher from Poland and spends a lot of time out at sea. "Everyone thinks there's nothing special there, but down below it's either all great and rosy（美好的） or it's a tragedy（悲剧）." Shipwrecks（海难） from the Second World War, bombs, munitions and toxic（有毒的） chemicals pose（形成） a risk to the ecological（ 生态的） balance of the Baltic（a波罗的海的） Sea, threatening（威胁） life in it and around it. Benedict Hutch is officially retired. As a former（在前的） commander（司令官） with the Polish Navy, he spent much of his life on the water.

  Now he's researching what to him is home, the Baltic Sea. He may be in his mid-sixties, but he's not planning to put his feet up just yet. "It appears we're now in the first phase（月相） of discovering the secrets of the Baltic（a波罗的海的） Sea. We already know a great deal, but we're still searching for new dangers and we're still discovering them. That's sad." Benedict Hutch and his crew（全体船员） are approaching（向…靠近） yet another wreck（失事）, a potential（潜在的） hazard（冒险） to the environment. They scan it using sonar（声纳）, a kind of underwater radar（[雷达] 雷达）.

  "We're now sailing over the wreck. Yes, exactly. We're now sailing over the hull（(果实等的)外壳） of the submarine. It's also visible（可见的） on the sonar（声纳）, but because the waves are so strong, the image isn't perfect. It looks as if the wreck has been torn（ 撕裂） apart, but it's still in one piece. You can see a straight line, a straight hull." All clear.

  The wreck is known and not dangerous. "I reckon（ 推断） there are about 5,000 wrecks（失事） in Polish waters. We've only explored about 20% of the ocean floor in Polish territory（领土）." Which still leaves 80%, and hundreds, perhaps thousands of shipwrecks lying on the bottom of the sea. Benedict Hutch leads his expeditions together with a research institute（研究所）. Their purview is the hull of the Baltic Sea. Their home is Gdansk Bay（湾）.

  This is the place where the Second World War was started by Nazi Germany under Adolf Hitler. Its ships were sunk in the final years of the war, as well as tons（吨） of munitions. Benedict Hutch discovered one of the most toxic wrecks, the Stuttgart. It was once an elegant（ 文雅的） passenger steamer（轮船）, then used as a hospital ship for Hitler's navy. It was sunk by American forces in the fall of 1943. Its steel（钢） hull（(果实等的)外壳） has been at the mercy（仁慈） of the seawater for almost 80 years.

  "Anyone who's ever had a Polish car knows what happens when the rust（锈） attacks. Even ships and wrecks decay（衰退）. The construction（建造） just breaks apart." The sonar image shows the wreck of the Stuttgart, an expansive debris（碎片） on the seabed（海底）. But what Hutch found around the ruined（毁灭） ship is even more worrying. The researchers took samples from the sea floor and brought them up to the surface.