Ong's Hat is one of the earliest Internet-based secret history conspiracy theories. It was created as a piece of collaborative fiction by four core individuals, dating back to the 1980s, although the membership propagating the tale changed over time. Ong's Hat is often cited as the first ARG on many lists of alternate reality games.
Related: Ongs Hat - The-Beginnning - by Joseph Matheny (PDF)
Related: Ong’s Hat creator Joseph Matheny on the blurred lines between fiction and reality