You are at war. Trains are key to keeping your army supplied with fuel, ammunition, food, and medical supplies. But, inexplicably, your trains keep blowing up. Sabotage? Enemy attack? There’s no evidence of a bomb or overt enemy attack. This is the situation the German military found itself in during World War II. As you can see in the video below, the hidden bomb was the brainchild of a member of Britain’s SOE.The idea was to put plastic explosive inside a fake plastic lump of coal. They hand-painted each one, and the color had to match the exact appearance of local coal. Paint and coal dust helped with that. The bomb had to weigh the correct amount as well.The coal was safe until it got quite hot, so resistance fighters could easily carry the coal and surreptitiously drop the bomb anywhere coal is stored. Eventually, it will be put in a boiler, and at the right temperature, it will do its job. There’s some actual footage of a test in the second video below.As the CIA notes, the idea actually dates back to the US Civil War. [Thomas Edgeworth Courtenay] built coal “torpedos” in the 1860s. (In those days, a torpedo could refer to any kind of bomb.) Probably the biggest impact was to tie up soldiers to guard coal stocks. However, in 1864, the USS Chenango’s boiler exploded in New York, and although the Union denied it, [Courtenay] was convinced it was one of his coal torpedoes that had done the trick. Later that year, Greyhound, the personal steamer of Major General Benjamin Butler, exploded right after taking in fresh coal. The CIA also mentions how coal bombs were also produced by the OSS, and even the Axis powers had their own version.While we are no fans of war, we have to admit we are always fascinated with war technology. Even if that means microwave death rays. Certainly, hiding explosives in coal qualifies as a wartime hack.