A World War II American naval mine was recently found in the straits that separate the Japanese islands of Honshu and Kyushu. It was an American MK25 naval mine, one of thousands delivered by submarine and air in 1945, in a successful effort to completely shut down Japanese shipping. These were all influence mines, meaning that they were built to detonate when metal, water pressure, or noise were detected. In those days, each mine did not have all three sensors and a mix of mines with different sensors were usually planted in crucial areas.
One reason that mines were so effective was that much of Japanese shipping was actually carried by very small vessels along the shallow coastal waters. At first the U.S. used aircraft and warships to try and interdict this traffic. Much of Japan's foodstuffs was moved in small craft of 80 tons or less. These small ships avoided the warplanes and warships by running along the coast at night and hiding in bays, rivers, and other inlets by day, where they could be camouflaged against American aircraft. Mines shut down this coastal shipping and the Japanese could find no effective way to deal with the many “weapons that wait.” Some of those mines are still waiting.
When the U.S. began to mine Japanese waters in 1945, the effects were devastating. Because it was so expensive to build roads and railroads in mountainous Japan, much of the domestic transportation was via small coastal freighters. These ships were more often sunk, rather than damaged, by mines. Once this coastal shipping system was shut down, essential items like food and fuel could not be moved. As a result, Japan began running short of food. Had Japan not surrendered in August 1945, millions of Japanese would have starved or frozen to death before the warm weather arrived in early 1946.
The Japanese Navy was called in to deal with the recently discovered old mine. While elderly, these mines are often still viable (able to explode) and for that reason they are, whenever possible, destroyed where they are found. The explosives were carefully placed near the MK25 and detonated by remote (very remote) control. The explosion of the 545 (1,200 pounds) of explosives in the 880 kg (1,935 pound) MK25 sent up a pillar of water a hundred meters (310 feet) high.
Some 25,000 naval mines were placed (by aircraft or submarine) around Japan in 1945. Only about half were cleared after the war. Much of this work was done by demobilized Japanese sailors operating their old mine clearing ships as civilian contractors. The clearing was greatly aided by the U.S. providing the general location of all the mines put in place by aircraft and subs. Half the mines, that were dropped in rarely trafficked waters, were left in place because by the end of 1945 their sensors no longer worked and they were no longer a danger unless violently disturbed. Since this was the pre-GPS era, a lot of the mine locations were approximate and over the following decades tides, currents, and storms moved a lot of these mines (all in shallow coastal waters) and some ended up in more heavily used waters where they are being rediscovered. If one of these old mines is discovered in shallow, well trafficked waters, they have to be destroyed before a ship (or a ship anchor) disturbs it. Sport divers are another danger, even though local divers are usually warned about old mines.
There are many other old World War II explosives found off Pacific islands. Three years ago, some 800 meters off the coast of Okinawa, a phosphorus bomb drooped over sixty years ago finally went off. Without any human intervention. The bomb had been dropped into shallow coastal waters in 1945, when U.S. troops invaded the Japanese island. Decades of tidal action and storms moved the bomb to shallower waters, until, exposed to the air, the phosphorous ignited, surprising people on the nearby beach with a column of white smoke. Japanese bomb disposal teams showed up to deal with it and found another unexploded bomb nearby, as well as a 105mm artillery shell. This was not the first time such a delayed bomb went off, but the last occurrence was 38 years ago and there may not be another. But there will be unexploded bombs, shells, and grenades found on Okinawa for decades to come.
Meanwhile, there are plenty of other Pacific battlefields where bombs are also being found. Three years ago, on another Pacific island (Guam, a U.S. territory), construction workers discovered a World War II era thousand pound (455 kg) bomb, when their backhoe hit it. The bomb didn't go off, and bomb disposal technicians determined that it was safe to leave it alone until the weekend, when they would try to remove the fuze and then move the bomb. If the bomb could not be moved, it would be detonated where it was found. A bomb that size has about 295 kg (650 pounds) of explosives. Thus when the bomb technicians went to work on the bomb, all people living or working within a thousand meters (3,100 feet) had to move so they are at least 1,600 meters from the bomb. Or move away at least 1,000 meters and stay indoors while the defusing was underway.
The bomb disposal teams on Guam are still called out 4-5 times a week, seven decades after World War II ended. It's worse in Europe, where hundreds of World War II explosives are unearthed each year in Germany alone. Usually there are no casualties, as bomb disposal technicians are well trained and get lots of practice. But the fuzes that did not go off in the 1940s are now getting old and more prone to detonation while being disabled. Detonating bombs in place is often expensive because it means evacuating lots of people and exposing homes and businesses to bomb damage.
It’s not just aircraft bombs. Most of the explosives unearthed are smaller items like grenades, mortar shells, rockets, and mines. Many bombs, artillery, and mortar shells (over ten percent, for some manufacturers) did not explode when they were supposed to but just buried themselves into the ground. These shells are still full of explosives and often have a fuze that, while defective, is often still capable of going off if disturbed. Other munitions were left in bunkers, or elsewhere on the battlefield, and got buried and lost. Most of these lost munitions eventually get found by farmers, or anyone digging up the ground for construction. Most large cities, in Europe and the Pacific, that were heavily bombed during World War II still suffer from construction crews unearthing unexploded bombs.
The problem goes back farther than World War II. Unexploded munitions from the World War I (which ended in 1918), and the American Civil War, which ended in 1865, are still showing up, and some of them are still deadly. Currently, over a thousand World War II munitions are discovered each year in Europe.