It's called that because they meet without state approval, CBS News correspondent Barry Petersen reports.
The church let CBS News in to take pictures that show both faith and defiance, while the congregation was brave enough to share their stories of what happens when the government finds the faithful.
"The police called us evil and arrested us for illegal assembly," one woman explains.
Chinese-born American Dr. Sam Chao works in China for religious freedom and researched the dangers faced by underground church worshipers.
"I interviewed about 40, 50 people; every one of them (was) beaten and jailed and harassed and beaten on the body," says Chao, Director of China Ministries International.
When Mao Tse-tung took power, he banned religion. Communism was the new faith. These days, the state allows churches, but worshippers must register with police. And yet, there are many young people worshipping.
"They've grown up in a society where the moral imperative is to make money and get ahead, and that doesn't take you very far," says Ken Lieberthal, a China expert at the University of Michigan.
People in Beijing are discovering perhaps a simple truth: Money alone is not bringing happiness. Hundreds of millions — and more each day — are seeking something more.
So even traditional eastern religions like Buddhism are flourishing again. But none like the underground Christians.
In one village, they were even building their own church … until the authorities knocked it down.
In China, it was once easy to know what to believe in — the Communist Party. Those days may be gone for good as a new generation learns how to find and keep its own faith.