Sweden's military is searching for "foreign underwater activity" less than 31 miles from Stockholm.
According to Reuters, the mobilisation of troops, ships and helicopters is on a scale not seen since the depths of the cold war.
Russia's bellicose actions in Ukraine are making nations in the Baltic and Nordic regions uneasy. Stockholm is just across the Baltic Sea from a major Russian naval base in Kaliningrad (see map above), a port in the Russian enclave between Poland and Lithuania.
According to the Swedish military, the information about the suspected Russian sub came from a trustworthy source. Swedish newspaper Svenska Dagbladet has said that the search was precipitated by a radio signal sent on an emergency Russian frequency.
Captain Jonas Wikstrom, head of operations for the search, said:
We still consider the information we received as very trustworthy. I, as head of operations, have therefore decided to increased the number of units in the area.
Stockholm is in an archipelago (second map) and ships and helicopters have been combing the area in scenes reminiscent of the cold war. In 1981 a Soviet submarine was discovered in Swedish waters, despite Sweden being neutral country.
Last month two Russian aircraft entered Swedish airspace. The Swedish government complained to Moscow, calling the incursion a "serious violation."