In May 1945, a pastor from Bly, Oregon, led his wife and a group of children on a day trip near Klamath Falls. They were all looking forward to hours of fishing and picnicking in fine weather. Everyone got out of the car while the Rev. Archie Mitchell was parking along a remote logging road and unloading the fishing tackle. Suddenly, he heard his wife, Elsie, who was five months pregnant, call out: “Look at what we’ve found! It looks like some kind of balloon.”

Those were her last words.

An explosion ripped through the mountain stillness, sending twigs, branches and sawdust flying into the air as the blast echoed through the mountains. Along with Elsie Mitchell, it took the lives of Sherman Shoemaker, Edward Engen, Jay Gifford, Joan Patzke,and Dick Patzke, all aged between 11 and 14 years old. These were the only enemy casualties of a Japanese military campaign to attack the United States with a most unusual, brilliant and, ultimately, ineffective weapon: balloon bombs.

The fūsen bakudan (or fu-gō, as they were commonly called) were high-altitude explosive and incendiary devices that were sent over the Pacific toward North America. With no targeting system, they landed at random. The deadly payloads that fell silently, haphazardly from the skies above were designed to terrorize the population. Initial reports about the inscrutable weapons fueled speculation that Japanese forces were landing on the continental U.S. — but, soon, all news of the strange objects stopped.

Japan’s balloon bombs remain little known 70 years after the end of World War II for several reasons. They were developed in strict secrecy by the Japanese military as its naval fleet suffered a crushing blow in 1944 and could no longer strike the United States. The U.S. government also censored virtually all news reports of balloons striking its territory, threatening to charge those who did disseminate such news with aiding the enemy. The War Department destroyed much of the evidence of the bombs. Finally, the bombs did very little damage compared to the scale of the conflict.

But the explosive balloons were remarkable feats of engineering with a distinctly Japanese touch. Their development was centered at the Imperial Japanese Army Noborito Laboratory, located in the hills of Kawasaki southwest of Tokyo on land in Kanagawa Prefecture that now belongs to Meiji University. Known for its links to the military Unit 731, which experimented on human subjects in Harbin, the lab was charged with developing secret weapons and techniques to undermine enemy states, such as the production of counterfeit currency distributed in China. In response to the April 1942 “Doolittle Raid” on Tokyo — the first U.S. attack on Honshu — Japan wanted to hit back by any means possible. The army considered initial plans to load high-flying balloons with the rinderpest virus, but this was ultimately abandoned for fear of a terrible retaliation by the U.S. — which came anyway in the form of the atomic bombings of Hiroshima and Nagasaki.

What made the balloons ingenious was their design and deployment. The main part was a hydrogen-filled balloon up to 10 meters wide that would carry explosives and incendiaries far across the Pacific to enemy territory 10,000 km away. Materials were scarce in Japan during the latter part of the war, and so the military engineers hit upon the idea of using traditional Japanese washi paper, produced from the fibers of mulberry trees. Washi manufacturers throughout the nation were mobilized to supply paper for the project. Schoolgirls were also recruited in the production effort so they could use their small hands and manual dexterity to assemble the dozens of sheets of paper needed to create a single balloon, and glue them together with paste from the konnyaku, or “devil’s tongue,” plant, which was designated war materiel. Photos from the era show girls dressed in monpe work pants and aprons, working over vats containing the paper sheets or even rolling an enormous paper balloon inflated with gas. For a military weapon, the papermaking technique may have been crude but it kept the hydrogen gas from escaping, even at very high altitudes with low air pressure.

That’s where the genius of the fu-gō came in — it was a weapon that could soar far above the land and sea, using a force of nature that had never been exploited before. In the 1920s, Japanese meteorologist Wasaburo Oishi conducted a series of experiments with pilot balloons launched from various locations in Japan. His accurate recordings from Tsukuba, northeast of Tokyo, led him to conclude that there were persistent, high-altitude winds blowing from the west, at least during the winter and spring in Japan. When the military gained control of his aerological observatory in the 1930s, Oishi’s data was “instrumental in planning the balloon bomb program,” John M. Lewis of Nevada’s Desert Research Institute wrote in the Bulletin of the American Meteorological Society. Japan may have lacked the B-29 Superfortresses that would level its cities, but it had a new way to attack its foe: the jet stream. This high-altitude, high-speed conveyor belt could be used to send bombs and terror to America, the military engineers reasoned. All they needed was the right design.

Suspended from the balloon was a “chandelier” consisting of the payload, ballast and a mechanism to regulate the balloon’s altitude. The mechanism had a device connected to a group of sandbags hanging from the chandelier. As the balloon rose to about 10,000 meters altitude, it would drift across the Pacific Ocean, a crossing that would take about four days. But at night, the air would cool and cause the hydrogen gas to shrink, which would, in turn, make the balloon lose altitude and threaten to dunk it in the sea. That’s where the timing device came in. It would be set to trigger a small charge after a number of hours, as the balloon was losing altitude, and the blast would cause a number of sandbags to fall away into the sea. The lightened balloon would then climb back up. Over the course of a few days, the balloon would describe an undulating path in the rarified air over the ocean. After its payload was dropped, the balloon even had a self-destruct mechanism to prevent reverse-engineering — a demolition block beside the battery box on top of the chandelier contained a picric acid explosive that would destroy the control mechanism and much of the chandelier itself.

Even the release valve of the balloon, which had a spring-loaded gasket mounted on a three-legged bracket, earned the praise of U.S. military authorities. A 1945 U.S. Navy training film entitled “Japanese Paper Balloon” describes it as “an extremely ingenious device for maintaining a fixed altitude of 30,000 feet (9,144 meters) and for dropping the payload.”

The film, available on YouTube, is one of the most detailed sources of information about the balloons, describing their 64 rope shrouds about 40 feet long (12 meters), the automatic ballasting and firing control, and the payload of up to four 10-pound (4½-kg) incendiaries and a central charge of about 60 pounds (27 kg) of anti-personnel explosives. The film goes into considerable forensic detail regarding the balloon’s mechanism, including its lead-acid wet-cell battery, aneroid barometer, and electrical and fuse circuitry. The effort to reverse-engineer the balloons reflects the U.S. government’s alarm over them when they began showing up everywhere from Alaska to the Mexican border in 1944 and 1945.

“In many ways, it was the first intercontinental weapons system,” says Jeff Noakes, a World War II historian at the Canadian War Museum in Ottawa, which has a balloon bomb among its exhibits. “The mechanism that kept the balloon at the appropriate altitude so that it kept travelling in the jet stream and that, ultimately, released the incendiary and high-explosive bombs had to work in a hostile, high-altitude environment, with very low temperatures and low air pressure.”

While the balloon bombs were the result of work by numerous branches of the Japanese military, government and private sector, they also involved teenage recruits at the Noborito Lab. Enji Ota, now 87, was one such recruit. Because his two brothers were fighting in the war, he took a job as an apprentice at Noborito so he could support his mother. The project he was ordered to work on was shrouded in secrecy, but Ota realized it involved balloons of some sort. Only 16, he had no idea that the balloon prototypes he helped test on chilly mornings in Kazusa-Ichinomiya, Chiba Prefecture, in 1944 would eventually be sent toward North America.

“I feel very sorry for the children who died because of one of the balloon bombs, but Japanese people also suffered terribly in U.S. air raids on Japan,” Ota, speaking in a raspy but steady voice, said recently during a press tour at the Noborito Lab, where he worked from April 1943 until August 1945. In the final months, as the situation in Japan became desperate with inevitable defeat, Ota helped destroy evidence of the balloon bomb program.

“War is a miserable thing and it must never again be waged,” Ota added ruefully.

The Americans from Bly weren’t the only ones killed by the balloons. Six Japanese also died in an accident when releasing them, according to Akira Yamada, curator of the defunct Imperial Japanese Army Noborito Laboratory Museum for Education in Peace. The museum is housed in an original wartime laboratory building and has a small-scale replica of a balloon bomb as well as exhibits about how the washi paper was fabricated.

“The attacks by balloon bombs were adopted as a last-ditch measure,” says Yamada. “Imperial Japanese Navy aircraft carrier units didn’t have enough fuel to attack the American mainland, and the military didn’t have any long-range bombers that could do the job.”

About 7 to 10 percent of the 9,300 bombs sent toward North America are believed to have survived the transpacific crossing and about 300 are confirmed to have landed, according to Ross Coen, author of “Fu-Go: The Curious History of Japan’s Balloon Bomb Attacks on North America.”

Coen relates how a sheriff in the Salt Lake City area responded to a farmer’s discovery of a balloon in his field by rushing out and tackling the strange object with his body. A chance gust of wind caused the balloon to lift the sheriff aloft, dangling above the explosive payload, over a canyon. After an exhausting struggle in which he was buffeted around, up and down with the wind, the lawman finally managed to grab a sagebrush root when the balloon lost altitude, and used it as an anchor to wrestle the balloon to the ground. He later received a letter of thanks for his hard work from then-FBI Director J. Edgar Hoover.

But while the balloon bombs were never a real threat to Canada and the U.S., they couldn’t be discounted entirely.

“The bombs did succeed in tying up American and Canadian resources used for countermeasures,” Noakes says. “These included teams of specialists to respond to reports of landed balloon bombs, aircraft used to patrol against and attempt to intercept incoming balloon bombs, and measures to be ready for forest fires caused by the bombs.”

Meanwhile, the deadly washi spheres scored a propaganda victory for Japan at a time when it was much needed.

“The balloon bombs were also intended to help sustain morale on the home front in Japan, since they were a way of striking at North America,” says Noakes. “A number of propaganda broadcasts were made after the few early reports of balloons reaching the United States were published (in this case, the Japanese military received the news via a Chinese newspaper that had re-published news about a mysterious balloon found in Montana). These broadcasts portrayed the balloon bomb campaign as successful and devastating, and promised even more impressive attacks on the United States.”

Aside from being a technological curiosity of the history of World War II, the balloon bombs continue to pose a remote but real threat to North America, just like unexploded ordnance in Europe, Japan and other territories devastated by war. Dozens or hundreds of balloon bombs could still lie undiscovered in Canada and the U.S. In October 2014, two loggers stumbled across the remains of one of the balloon’s payloads in a remote wooded area in the Monashee Mountains of British Columbia. A Canadian Navy bomb disposal unit confirmed the partially buried metal casing was a wartime balloon bomb, judged it too dangerous to excavate and blew it up with C4 explosive. The remains of the bomb, a failed attempt to strike a distant foe, were scheduled to be donated to a local museum.

As for Elsie Mitchell and the five children who died when they stumbled upon the balloon bomb in Oregon, the 1950 Mitchell Monument in the Fremont-Winema National Forest memorializes the tragedy with the words: “Only place on the American continent where death resulted from enemy action during World War II.”