Japanese Balloon Bombs
Many schoolchildren in America grow up believing that Japan only attacked American soil once, in the attack on Pearl Harbor, and never attacked the US mainland. This is simply not true.
On April 18, 1942, the US Aircraft Carrier Hornet was travelling towards mainland Japan with orders to send out its planes for a sneak attack on Tokyo once they got at least 650 miles away from the capital city, allowing the B-25 bombers to drop their payloads and land on friendly Chinese airstrips. As it happened, however, the Hornet encountered a Japanese Patrol Boat, which they sank, but were unsure as to whether the Patrol boat alerted the Japanese forces to their sneaky arrival.
The decision was made to send the sixteen bombers, even though they were still 800 miles away from the target. The B-25s all completed their mission, but none of them were able to make it to China, with most of the Navy airmen ditching their planes in the Ocean, or crash landing on Japanese soil.
However, it was the Japanese retaliation to this attack that was interesting. In response to the Doolittle Tokyo Raids, as they were called, the Japanese created from 9,000 to 15,000 giant balloons (FUGOs) which were strapped with explosives. These balloons, 32 feet in diameter when inflated with 19,000 cubic feet of hydrogen, were first launched on November 3, 1944.
The way the balloon bombs worked was that they were inflated with enough hydrogen so that they rose to a height where they could be carried by the jet stream to American soil, somewhere in the range of 20,000 to 40,000 feet above sea level. The balloon would gradually drop as it lost hydrogen and gathered dew. However, once the balloon fell below 20,000 feet, it would trigger the release of a sandbag, which would allow the balloon to regain altitude and rejoin the jet stream. Eventually, the sandbags ran out, and they were timed so that when the balloon lost all its sandbags, it would be over America, and would crash to the ground, exploding its carried bomb.
The US Government did not publicize the danger, hoping to make Japan think their attack was a failure, but after six church picnickers in Bly, Oregon were incinerated by a bomb they found in the forest, the Government quickly let the populace know of the dangers of the devices.
The most imminent dangers from the balloons were the threats of using them as vehicles for germ warfare and forest fires, although there were no reports of Japan using disease-filled balloons. To counter the threat, Navy and Air Force pilots shot down many of the balloons, and Army stations were set up to counter the possibility of fires and germ warfare.
Thanks to the silence about the bombs, the Japanese were unaware of their successes (about 300 balloon bombs made it to American soil), and discontinued the program in April 1945.
Seattle Times story of the six picnickers:
(image from http://www.cufon.org/cufon/5004fugo.htm)
 United States Air Force Museum. Japanese Balloon Bombs http://www.wpafb.af.mil/museum/history/wwii/jbb.htm March 14, 2004.