Two million people descend on Daecheon Beach for ten days of mud wrestling, mud sliding, mud swimming, and general mud-based chaos. The mud from the Boryeong mudflats has genuine cosmetic properties — high in minerals and bentonite — which is the original rationale for what has become one of Korea's biggest summer festivals. There are mud pools, mud prisons (where you pay to be trapped in mud with strangers), mud fireworks, and mud everything. It's gleefully absurd, extremely social, and one of those events where everyone ends up as new friends by the end of day one.
Buses from Seoul's Express Bus Terminal run directly to Boryeong — about 2.5 hours. The festival peaks on weekends. Bring a change of clothes and expect to spend time at the outdoor showers.