July 8, 2007: A British mine-sweeper, the HMS Penzance was out training last month, near an oil terminal in the Shetland islands. Searching the sea bottom with its special sonar, what should it discover but a World War II era mine, not 700 meters from the terminal. It was a 500 pound contact mine, covered in barnacles. An explosive charge was placed near the mine, and detonated. There was not much of a secondary explosion, indicating that most of the explosives in the mine had deteriorated. Many World War I and II naval mines were loaded with cheap explosives, because of wartime shortages.

Several hundred thousand mines were laid in the North Sea during the two World Wars. Most were swept and destroyed after the wars, but hundreds are still unaccounted for. For about a decade after each war, there were incidents of these mines washing ashore, or being spotted at sea. Now, these old mines have largely settled on the bottom, often too far down to be a danger to anyone. But those that came to rest in shallow water, at least provide practice for mine hunting boats.