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.