In 1981, S-363 grounded about 6.2 miles from the Karlskrona naval base while Sweden was testing new anti-submarine equipment. Its presence during those trials raised suspicions of Soviet intelligence gathering. An unarmed Swedish naval officer boarded the submarine and demanded an explanation for its presence inside territorial waters. The Soviet captain blamed simultaneous failures of the navigation equipment. That explanation is hard to reconcile with the narrow, rocky approaches, which a submerged boat could not safely negotiate without working navigation. S-363 had also tried to escape on the surface and had sent no distress signal before the weather worsened. Swedish destroyers remained nearby as visible evidence of Sweden's capability and intent. The captain later came ashore for questioning with Soviet diplomatic representatives present, accepting the interview under immunity.
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S-363 aground near Karlskrona
Soviet submarine S-363 grounded near Karlskrona during a Swedish anti-submarine exercise, turning a long-running security concern into a visible Cold War confrontation.