In 1940, the NOL decided to copy the mechanism of a German magnetic mine received from the British. Since the mechanism required a nonmagnetic case, the United States manufactured replicas of the German magnetic mine mechanism and placed them in aluminum mine cases. These devices were used early in World War II as Mk 12 mines.

Mines in the Pacific Theater

In October 1942, mines were laid in approaches to Bangkok, Thailand; Haiphong, (North) Vietnam; and the Hainan Strait, south of China. The mines, laid by submarines, immediately sank six ships and damaged six more. A total of 421 mines, planted by submarines in 21 areas, sank 27 ships and damaged another 27 ships.

About the same time that the Mk 12 mine was developed, the NOL initiated designs for several new mines that would respond to magnetic, acoustic, and/or pressure influences of ships. Other influences (such as gravitational, optical, cosmic ray, and electrical) were considered impractical and, therefore, were rejected. The mines were designed to be launched from aircraft and to lie on the bottom after planting. Other features of these mines included (1) various sensitivity settings, (2) clock delay arming devices, (3) electrolytic sterilizers, and (4) ship counters.

The Mk 36 series mine contained 500 pounds of high explosives, while the Mk 25 series mine contained 1,000 pounds. Under the direction of the BUORD, the NOL started production of both mines in 1941. In 1944, the mines began to be delivered in quantities.

The Mk 25 and Mk 36 mines were used against the Japanese during Operation Starvation in the last 4 months of World War II. To carry out the operation, the Navy turned to the Army Air Force for the B-29 aircraft, which had the capability of carrying twelve 1,000-pound mines to destinations as far away as 1,500 miles (3,000 miles round trip).

On 22 December 1944, the Army Air Force issued orders for mining operations to begin on 1 April 1945. After the order was issued, the Navy moved a team of mine experts to Tinian Island in the Mariana Islands. One month later, the Navy had a mine assembly depot completed and in operation on the island.

The first minelaying in that operation occurred on 27 March 1945. The mines were planted in the Shimonoseki Strait, between the Japanese islands of Honshu and Kyushu. A total of 12,000 mines were planted in and around the Japanese main island of Honshu, and an additional 13,000 mines were planted in harbors and channels surrounding the newly extended Japanese Empire.

Mines in the Atlantic Theater

The British and the Germans also used mines during World War II, with devastating results. The British vessels sunk by German mines included 280 combatant and 296 merchant ships. The German fleet lost 250 combatant and 800 merchant ships to British mines. German mines destroyed 22 percent of the landing craft lost by the United States and British navies in landing operations in 1944 and 1945.

The Germans even planted 266 mines in the Atlantic Ocean along the eastern seaboard of North America, including 36 mines off Charleston, South Carolina. Nine ships were either sunk or damaged as a result of these mines.