Zion, Illinois was the American center of Flat Earth theory in the early-to-mid 20th Century. It was a small community (roughly 6,000) focused largely on the Christian Catholic Apostolic Church, headed at first by John Alexander Dowie and later by Wilbur Glenn Voliva, the latter being one of America's most prominent Flat Earth proponents in the 20th Century. Zion reached its peak in the Flat Earth movement in the 1920's and 1930's, although interest began to wane after Voliva's death in 1942.

Zion remains a small city of roughly 25,000 residents. The Christian Catholic Apostolic Church has since been renamed the Christ Community Church and functions as a Protestant Evangelical church.

http://www.bringyou.to/apologetics/p43.htm wrote:
In America flat-earthism became a central doctrine of Wilbur Glenn Voliva's Christian Catholic Apostolic Church in Zion, Illinois. During the 1920s and 1930s, thousands of residents of Zion were at least nominally flat-earthers. In some families three generations learned the flat-earth doctrine in Zion parochial schools. From his 100,000 watt radio station, Voliva used to thunder against "the Devil's triplets, Evolution, Higher Criticism, and Modern Astronomy." The popularity of flat-earthism declined in America after Voliva's death in 1942