 Preparations were made in winter 1942 to prevent the intrusion of Soviet submarines in the open Baltic for the following year.A decisive element of the Axis blockade was laying a double anti-submarine net, named "Walrus" system, across the Gulf of Finland from Porkkala (Finland) to Naissaar (Estonia).Additionally, the German Navy laid 7,293 other new mines, while Finland laid 1,965 mines, in addition to the existing ones.