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On a rainy afternoon last Spring, American pastor Eric Foley and his wife stood in a muddy field near the North Korea border and prayed – their hands clasped to a 40-foot homemade balloon that would carry Bibles to the communist dictatorship's underground Christians.

"I get choked up, every time, as I let go and watch it take off," Foley told FoxNews.com.

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The balloons, made from a large sheet of "farm plastic," said Foley, are filled with hydrogen before the Bibles and "tracts" – testimonials written by other North Korean Christians – are attached at the bottom inside a sack or box. Timers are then used to release the materials in stages, dispersing them at high altitudes across North Korea. Foley and members of his Christian mission group, Seoul USA, use GPS technology to help direct where the Bibles land. Around 50,000 of them have dropped from the skies in the last year.

"They are the most persecuted believers on earth," Foley said of North Korea’s estimated 100,000 Christians – 30,000 of whom are believed to be locked inside concentration camps, where they are overworked, starved, tortured, and killed. Other activist groups, like Open Doors USA, estimate that number to be even higher, reporting that the secretive nation has about 400,000 Christians.

In North Korea, the practice of Christianity is illegal. Owning a Bible is a crime, and any person caught with one is sent – along with three generations of his or her family – to prison. Foley said despite the risks, demand for Bibles is strong within North Korea. His group targets rural areas where they might be picked up discreetly, he said.

North Koreans are forced to embrace Juche ideology, which mixes Marxism with worship of the late "Great Leader" Kim Il Sung and his family – a warped version of Christianity, says Foley, because Kim took concepts from Christianity, like the Trinity and church hymns, to create a religion in which he is worshipped. Foley said that if North Koreans learned about Christ, they would realize "this is all a fraud."

"It's a distortion of Christianity," Foley said. "And the best way to reach them [North Koreans] is through mindset and knowledge."

Foley, who is in his late 40s, founded Colorado-based Seoul USA in 2003 with his wife, a South Korean who immigrated to the U.S. in 1984. The two, along with other members of their group, launched their first balloon -- strapped with Bibles -- from South Korea in 2006. Foley said the balloons are typically sent out overnight from a muddy field at a high altitude between May and October. He said the best conditions are during a "rain storm or really bad weather because of the currents."

"We are constantly monitoring the wind conditions as we're launching," he said, "And the North Korean border is always within the sight line."

The balloons also include tracts, or testimonies, written by other North Korean Christians -- some of whom managed to flee to South Korea -- about Christ.

"The North Koreans respond very well to story," Foley explained, "Because all are required to memorize 100 stories" related to Kim's ideology.

In addition to supplying religious materials by air, Foley's group produces short-wave radio programs with North Korean defectors reading the Bible. He said about 20 percent of North Koreans own radios, which are illegal.

Foley and his group won the legal rights to conduct the balloon launches from South Korea, but officials there "don't make it easy," he said, noting that they often try to force hydrogen suppliers not to sell the group hydrogen.

"Every time we fill up one of these balloons, we hold it and we pray together in English, North Korean and South Korean," Foley said. "We pray loudly and always with tears."