submarine submarine

The conning tower of the ARA San Juan submarine shown as the vessel is being delivered to the Argentine Navy after an extensive refit in Buenos Aires, in May 2014. Alejandro Mortiz/AFP/Getty Images hide caption

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Alejandro Mortiz/AFP/Getty Images

Peter Madsen's private submarine sits on a pier in Copenhagen harbor. The Danish inventor faces a murder charge after Swedish journalist Kim Wall died aboard the sub under mysterious circumstances last month. Jens Dresling/AP hide caption

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Jens Dresling/AP

Peter Madsen's private submarine sits on a pier in Copenhagen's harbor. Danish police have identified a headless, limbless torso that washed ashore Monday as that of Kim Wall, the journalist who joined Madsen on his sub earlier this month to report a story. Jens Dresling/AP hide caption

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Jens Dresling/AP

Peter Madsen's privately owned submarine UC3 Nautilus, now a suspected crime scene, gets carried out of Copenhagen harbor on a truck for forensic police investigation earlier this month. Ole Jensen/Corbis via Getty Images hide caption

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Ole Jensen/Corbis via Getty Images

Helicopter belonging to the Netherlands participates in NATO's Dynamic Mongoose anti-submarine exercise in the North Sea, off the coast of Norway, on May 4, 2015. MARIT HOMMEDAL/AFP/Getty Images hide caption

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MARIT HOMMEDAL/AFP/Getty Images

The Swedish corvette HMS Visby is engaged in a military operation on the fifth day of searching for a suspected foreign vessel in the Stockholm archipelago, Mysingen Bay, Sweden, on Oct. 21. Swedish authorities say they have proof it was a foreign sub but don't know the nationality. Fredrik Sandberg/EPA/Landov hide caption

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Fredrik Sandberg/EPA/Landov

Swedish corvette HMS Stockholm patrols Jungfrufjarden in the Stockholm archipelago, Sweden, on Monday. Swedish authorities say they've detected "foreign underwater activity" thought to be a possible Russian submarine. Anders Wiklund/EPA/Landov hide caption

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Anders Wiklund/EPA/Landov