Editing Naval mine (section)

=== World War II ===
[[File:Bundesarchiv Bild 101II-MN-1559-33, Auf See, beim Minen legen.jpg|thumb|upright|A contact mine being deployed from the German minelayer {{ship|German minelayer|Hansestadt Danzig||2}}]]
During [[World War II]], the [[U-boat]] fleet, which dominated much of the battle of the Atlantic, was small at the beginning of the war and much of the early action by German forces involved mining [[convoy]] routes and ports around Britain. German submarines also operated in the [[Mediterranean Sea]], in the [[Caribbean Sea]], and along the U.S. coast.

Initially, contact mines (requiring a ship to physically strike a mine to detonate it) were employed, usually tethered at the end of a cable just below the surface of the water. Contact mines usually blew a hole in ships' hulls. By the beginning of World War II, most nations had developed mines that could be dropped from aircraft, some of which floated on the surface, making it possible to lay them in enemy harbours. The use of dredging and nets was effective against this type of mine, but this consumed valuable time and resources and required harbours to be closed.

Later, some ships survived mine blasts, limping into port with buckled plates and broken backs. This appeared to be due to a new type of mine, detecting ships by their proximity to the mine (an influence mine) and detonating at a distance, causing damage with the shock wave of the explosion. Ships that had successfully run the gantlet<!-- properly gantlet, not gauntlet --> of the Atlantic crossing were sometimes destroyed entering freshly cleared British harbours. More shipping was being lost than could be replaced, and [[Winston Churchill|Churchill]] ordered the intact recovery of one of these new mines to be of the highest priority.

[[File:L L sweeping gear.jpg|thumb|The towed, electric cables of ''Double-L'', magnetic–minesweeping gear being deployed behind a Royal Navy minesweeper]]
The British experienced a stroke of luck in November 1939, when a German mine was dropped from an aircraft onto the mudflats off [[Shoeburyness]] during low tide. Additionally, the land belonged to the army and a base with men and workshops was at hand. Experts were dispatched from {{HMS|Vernon}} to investigate the mine. The Royal Navy knew that mines could use magnetic sensors, Britain having developed magnetic mines in World War I, so everyone removed all metal, including their buttons, and made tools of non-magnetic [[brass]].<ref>Campbell, John, "Naval Weapons of World War Two" (London: Conway Maritime Press, 1985)</ref> They disarmed the mine and rushed it to the labs at HMS Vernon, where scientists discovered that the mine had a magnetic arming mechanism. A large ferrous object passing through the Earth's [[magnetic field]] will concentrate the field through it, due to its magnetic permeability; the mine's detector was designed to trigger as a ship passed over when the Earth's magnetic field was concentrated in the ship and away from the mine. The mine detected this loss of the magnetic field which caused it to detonate. The mechanism had an adjustable sensitivity, calibrated in [[Gauss (unit)|milligauss]].

[[File:Dwi wellington front.jpg|thumb|A [[Vickers Wellington]] fitted with a ''DWI'', magnetic mine exploder, [[Ismailia]], Egypt]]
From this data, known methods were used to clear these mines. Early methods included the use of large electromagnets dragged behind ships or below low-flying aircraft (a number of older bombers like the [[Vickers Wellington]] were used for this). Both of these methods had the disadvantage of "sweeping" only a small strip. A better solution was found in the "Double-L Sweep"<ref>{{cite web|url=http://www.goodeveca.net/CFGoodeve/cfg_bio.html#sweep|title=The Double-L Sweep – Biography of Sir Charles Goodeve|access-date=9 July 2008|archive-date=18 October 2008|archive-url=https://web.archive.org/web/20081018181128/http://www.goodeveca.net/CFGoodeve/cfg_bio.html#sweep|url-status=live}}</ref> using electrical cables dragged behind ships that passed large pulses of current through the seawater. This created a large magnetic field and swept the entire area between the two ships. The older methods continued to be used in smaller areas. The [[Suez Canal]] continued to be swept by aircraft, for instance.

While these methods were useful for clearing mines from local ports, they were of little or no use for enemy-controlled areas. These were typically visited by warships, and the majority of the fleet then underwent a massive [[Degaussing#Ships.27 hulls|degaussing]] process, where their hulls had a slight "south" bias induced into them which offset the concentration-effect almost to zero.

Initially, major warships and large troopships had a copper ''degaussing coil'' fitted around the perimeter of the hull, energized by the ship's electrical system whenever in suspected magnetic-mined waters. Some of the first to be so fitted were the [[aircraft carrier|carrier]] {{HMS|Ark Royal|91|6}} and the liners {{RMS|Queen Mary}} and {{RMS|Queen Elizabeth}}. It was a photo of one of these liners in New York harbour, showing the degaussing coil, which revealed to German Naval Intelligence the fact that the British were using degaussing methods to combat their magnetic mines.<ref>Piekalkiewicz, Janusz, "Sea War: 1939–1945" (Poole, UK: Blandford Press, 1987)</ref> This was felt to be impractical for smaller warships and merchant vessels, mainly because the ships lacked the generating capacity to energise such a coil. It was found that "wiping" a current-carrying cable up and down a ship's hull<ref>{{cite web|url=http://www.goodeveca.net/CFGoodeve/cfg_bio.html#wipe|title=Wiping – Biography of Sir Charles Goodeve|access-date=10 July 2008|archive-date=18 October 2008|archive-url=https://web.archive.org/web/20081018181128/http://www.goodeveca.net/CFGoodeve/cfg_bio.html#wipe|url-status=live}}</ref> temporarily canceled the ships' magnetic signature sufficiently to nullify the threat. This started in late 1939, and by 1940 merchant vessels and the smaller British warships were largely immune for a few months at a time until they once again built up a field.

The cruiser {{HMS|Belfast}} is just one example of a ship that was struck by a magnetic mine during this time. On 21 November 1939, a mine broke her keel, which damaged her engine and boiler rooms, as well as injuring 46 men, one later died from his injuries.  She was towed to [[Rosyth Dockyard|Rosyth]] for repairs.  Incidents like this resulted in many of the boats that sailed to [[Dunkirk evacuation|Dunkirk]] being degaussed in a marathon four-day effort by degaussing stations.{{sfn |Wingate|2004|pp=34–35}}

[[File:Laying mines aboard Finnish minelayer Ruotsinsalmi May 1942 (SA-kuva 88630).jpg|thumb|The Finnish minelayer ''[[Finnish minelayer Ruotsinsalmi|Ruotsinsalmi]]'' lays naval mines in the [[Gulf of Finland]] during the [[Continuation War]]]]
The Allies and Germany deployed acoustic mines in World War II, against which even wooden-[[hull (watercraft)|hull]]ed ships (in particular [[Minesweeper (ship)|minesweeper]]s) remained vulnerable.<ref name="Parillo, p.200">Parillo, p. 200.{{Incomplete short citation|date=May 2020}}</ref> Japan developed sonic generators to sweep these; the gear was not ready by war's end.<ref name="Parillo, p.200"/> The primary method Japan used was small air-delivered bombs. This was profligate and ineffectual; used against acoustic mines at [[Penang]], 200 bombs were needed to detonate just 13 mines.<ref name="Parillo, p.200"/>

The Germans developed a pressure-activated mine and planned to deploy it as well, but they saved it for later use when it became clear the British had defeated the magnetic system. The U.S. also deployed these, adding "counters" which would allow a variable number of ships to pass unharmed before detonating.<ref name="Parillo, p.200"/> This made them a great deal harder to sweep.<ref name="Parillo, p.200"/>

Mining campaigns could have devastating consequences. The U.S. effort against Japan, for instance, closed major ports, such as [[Hiroshima]], for days,<ref name="Parillo, p.201">Parillo, p. 201.{{Incomplete short citation|date=May 2020}}</ref> and by the end of the Pacific War had cut the amount of freight passing through [[Kobe]]–[[Yokohama]] by 90%.<ref name="Parillo, p.201"/>

When the war ended, more than 25,000 U.S.-laid mines were still in place, and the Navy proved unable to sweep them all, limiting efforts to critical areas.<ref name="Gilbert, p.5">Gilbert, p. 5.</ref> After sweeping for almost a year, in May 1946, the Navy abandoned the effort with 13,000 mines still unswept.<ref name="Gilbert, p.5"/> Over the next thirty years, more than 500 minesweepers (of a variety of types) were damaged or sunk clearing them.<ref name="Gilbert, p.5"/>

The U.S. began adding delay counters to their magnetic mines in June 1945.<ref>Parillo, Mark P. ''Japanese Merchant Marine in World War Two'' (Annapolis, Md. : Naval Institute Press, 1993), p. 200.</ref>