The Boryeong Mud Festival was first held in 1998. It started as a relatively small thing. It was probably lots of fun, but nowhere near as fun as it became.
By the year 2007, the festival was attracting upwards of two million people to Boryeong. That's a lot of people.
For perspective, Burning Man's attendance is capped at 70,000 people. The Boryeong Mud Festival is enormous.