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

itoggle caption 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

itoggle caption Anders Wiklund/EPA/Landov