On the Pacific island of 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. A bomb that size has about 295 kg (650 pounds) of explosives. Thus when the bomb technicians go to work on the bomb, all people living or working within a thousand meters (3,100 feet) must move so they are least 1,600 meters from the bomb. Or move away at least 1,000 meters and stay indoors while the defusing is underway.
The bomb disposal teams on Guam are still called out 4-5 times a week, 65 years after World War II ended. It's worse in Europe, where more than 2,000 tons 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.
Its 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, Europe and the Pacific, that were heavily bombed cities 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.