One of World War 2's best-kept secrets was that of the Japanese balloon bombs. Lacking a practical means to attack the US mainland during the war, the Japanese constructed 9,000 large hydrogen balloons, attached incendiary and anti-personnel bombs to them, and set them afloat on the high-altitude trade winds towards the United States.
It is estimated that about 1,000 of these weapons successfully crossed the 6,000 miles from Japan, a few drifting as far east as Michigan. Once the US military realized the nature of these balloons, they regularly shot them out of the sky, but adopted a policy of absolute secrecy to deny the Japanese any news of success. The news media cooperated with the military and didn't publish any news of incidents. After five months, the Japanese became discouraged by the bombs' apparent total failure, and halted the attacks. The policy of silence was so successful that few people have heard of these bombs even today.
In all, there were 285 balloon bomb incidents reported, and six deaths. All six deaths occurred when a group of picnickers in Oregon found a balloon bomb in the woods, and attempted to move it, causing it to explode.