It started with a manuscript originally named Excalibur back in (approximately) 1938, which then became the book titled Dianetics: The Modern Science of Mental Health, and was first released in parts via a magazine, but then culminated into a book that was then published in May 1950.

Due to high popularity of the book (actually 3 books in one), it hit the New York Times Bestseller list and stayed there for 28 consecutive weeks, selling over 150,000 copies within a year.(ref)

As the initial demand was so high, this prompted L. Ron Hubbard to continue further research into his discoveries of why the mind works as it does, and this later forced the development of Scientology, which was started in 1952, but became incorporated officially in 1953. (ref)

Scientology's popularity has been increased thanks to Hollywood, but their primary spiritual headquarters (or their biggest, depending on how you define it) is located in Clearwater, Florida. (ref)

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I found this previous answer from a similar question interesting.