The U.S. Navy has developed a technique, using superconductors, to rapidly degauss the hull of a warship, and make is less detectable by naval mines that sense large metal objects. This does not make ships immune to naval mines, but does make it more difficult for modern mines to detect their targets.
This is because current naval mines are "smart mines", containing its own computer, and able to detect targets via acoustic (detecting the sound of the target ship), pressure (detecting the pressure on the water a ship over head makes), and magnetic (detecting the metal in a ship) sensors. Many details of these bottom (they are placed on the sea bed of shallow coastal waters) mines are classified, and some are believed to have an electrical field sensor as well. Some can also be detonated by remote control. The computer in these mines are programmable, so the mine can be instructed to attack only certain types of ships. This is where the degauss technology becomes useful, because a smart mine will often go off only if a certain type of ship (setting off a specified number of sensors in a specified way) passes overhead. If the mine sensors do not detect the right combination of characteristics, they do not detonate. If the new degaussing technology becomes widely implemented, smart mines will have to be reprogrammed to try and deal with it.
While often ignored, naval mines, in general, are a formidable weapon. But they just don't get any respect. The historical record says otherwise. Modern naval mines were widely used for the first time a century ago, during the Russo-Japanese war (1904- 1905). These were contact mine, floating in shallow water and kept in place with an anchor and chain. When the tide was right, they would be just below the surface, ready to explode whenever struck by a ship. Some 2,000 of these mines were used to destroy sixteen ships during that first wartime use.
During World War I (1914-18), many modern mine tactics were developed. Thousands of mines were laid to provide defensive barriers against enemy movement in the North Sea. Mines were used offensively by secretly placing them across known enemy sea routes. More than 1,000 merchant and war ships were lost because of the 230,000 mines used. During World War II, a total of 2,665 ships were lost or damaged to 100,000 offensive mines. That's one ship for every 37 mines. Some 208,000 mines were used defensively to inhibit enemy movement and tie up his resources.
Naval mines achieved several striking successes during World War II. In the Pacific, naval mines proved more destructive to the Japanese war effort than the atom bombs. During a 10 week period between April and August 1945, 12,000 mines were delivered by American bombers. These destroyed 1,250,000 tons of Japanese shipping (670 ships hit, 431 destroyed). That's 18 mines for each ship hit. The Americans had air superiority, so losses during these 1,500 missions amounted to only 15 planes, most of them to accidents. Had these missions been flown against opposition, losses would have been between 30 and 60 aircraft, plus similar losses to their fighter escorts.
A conventional submarine campaign was also waged against Japanese shipping. Comparisons to the mine campaign are interesting. A hundred submarines were involved in a campaign that ran for 45 months from December, 1941 to August, 1945. Some 4.8 million tons of enemy shipping was sunk. For every US submarine sailor lost using submarine launched torpedoes, 560 tons of enemy ships were sunk. During the mine campaign, 3,500 tons were sunk for each US fatality. On a cost basis, the difference was equally stark. Counting the cost of lost mine laying aircraft (B- 29's at $500,000 each) or torpedo armed submarine ($5 million each), we find that each ton of sunk shipping cost six dollars when using mines and fifty-five dollars when using submarines. These data was classified as secret until the 1970s. It indicates that mines might have been more effective than torpedoes even if the mines were delivered by submarine.
The Germans waged a minelaying campaign off the east coast of the United States between 1942 and 1944. Only 317 mines were used, which sank or damaged 11 ships. This was a ratio of 29 mines used for each ship hit. In addition, eight ports were closed for a total of 40 days. One port, Charleston, South Carolina, was closed for 16 days, tying up not only merchant shipping but the thousands of men, warships and aircraft dealing with the situation. American submarines also waged a limited mine campaign in the Pacific. For 658 mines used, 54 ships were sunk or damaged (12 mines per ship). No subs were lost. Considerable Japanese resources were tied up dealing with the mines. On the Palau atoll, the port was closed by the mines and not reopened until the war ended. Even surface ships were used to lay mines. Three thousand mines were laid by destroyers. Only 12 ships were hit, but these were barrier fields, not the ambush type mine fields that a submarine can create by sneaking into an enemy held area.
In Korea during the early 1950s, the Soviets provided North Korea with 3,000 mines, many of 1904 vintage. These were used to defend Wonson harbor. It took several weeks for UN forces to clear these at a loss of a dozen ships hit. Half of these ships were destroyed.
During the Vietnam war, over 300,000 naval mines were used, primarily in rivers. The vast majority were not built as mines but were aerial bombs equipped with magnetic sensors instead of fuzes. These bombs/mines used a small parachute to insure that no damage occurred on landing. In shallow water these makeshift weapons sat on the bottom and performed as well as any other bottom mines. Haiphong Harbor was mined with 11,000 of these "destructors," as the US air force called them, and less than a hundred conventional mines. Haiphong Harbor was shut down completely for months, and it took years to clear out all the American mines. The "destructor" mine design was so successful, that it is still in use, using more modern electronics, as the Mk 62 mine.
During the 1991 Gulf War, the Iraqis laid over a thousand mines off the Iraqi and Kuwaiti coast. The predominantly US naval forces did not have sufficient mine sweeping resources to deal with this situation and had an amphibious ship and cruiser hit and damaged while trying to clear the area. This effectively prevented any US amphibious operations, although the Marines were not going to be used for a landing anyway. It took over a month of mine clearing after the fighting ceased to eliminate all the mines. In the meantime, two U.S. warships were damaged by these mines.
In any future war, naval mines will again surprise everyone with how effective they are. It is feared that terrorists might get their hands on some bottom mines, but so far, there do not appear to have been any attempts.