|During World War II at least 46 different mine types were detected in Gulf of Finland. These had about 10 different systems for triggering, including two magnetic and one acoustic types. The belligerent powers laid about 51 600 mines and 12 700 anti-sweeping devices during the War. Germany laid approximately 28 100 mines and 9 350 anti-sweeping devices. The rest was half and half by Finland and Soviet Union. In addition, Sweden laid almost 4 500 mines on its own territorial waters. In Gulf of Finland majority of mines in 1941 were laid on the mouth of the Gulf and off Juminda in Estonian waters. Later the mines were laid for anti-submarine purposes east from Suursaari and on the Porkkala-Naissaari line. Some offensive barrages were laid between Kronstadt and Lavansaari 1942-1944 and defensive barrages along coast of Karelian Isthmus, Koivisto islands and Bay of Vyborg in 1944.|
During World War II the Gulf of Finland was the most heavily mined area. After the war, by the order of the victors, clearing of the mines was Finlands duty. After the Armistice in 1944 some of the most important sea lanes were sweeped. These were mainly routes that Soviets needed to attack Germany. During a 40 days period over 700 mines, drifting mines and anti-sweeping devices were destroyed. Three vessels were lost. Twelve men were lost and nine wounded. During the main cleaning operation in 1945-1950 over 35 000 km2 was sweeped. 9276 mines or anti-sweeping devices were destroyed, about 10% of these were drifting. 28 men were lost and 37 wounded. This sweeping force was second largest in the world with 2 000 men and 200 vessels. Only Japan had a larger force.