The popularity of Korean cosmetics has exploded around the world in the past few years. Beauty junkies gush about the innovative formulas. Yet much of Korean skincare relies also on age-old ingredients such as honey, herbs and mushrooms. Another common one: mud.

Mud-made products are found in nearly every brand, whether it’s exfoliants that use the ancient volcanic soil of Jeju Island or charcoal clay masks designed to calm acne-prone skin. In fact, one of South Korea’s largest festivals, the Boryeong Mud Festival, was launched in 1999 to promote the beauty benefits of mud from the town of Boryeong, located a two-hour drive from Seoul on the country’s west coast.

Held on Daecheon Beach, Boryeong’s annual summer bash attracts the largest number of international visitors among the country’s local festivals. Attendees get down and dirty on the gloppy, grey beach, taking part in mud wrestling, sliding down inflatables and even swimming in giant mud baths. More athletic challenges include a 3K run across mudflats and a military-style training session. Those just looking to relax can head to the mud massage stations or take a dip in the sea. There are also parties backed by live music and fireworks, and a kids’ zone to keep little ones entertained.

But the biggest perk of rolling in the clay is skin deep. Just ask the town’s top official. "Boryeong city has a 136-kilometre coastline that consists of fine sea mud that is rich in the minerals germanium and bentonite," says Kim Dong-Il, mayor of Boryeong and head of the festival’s organising committee. "It also radiates a high level of far-infrared rays known to be beneficial for the skin’s appearance."

But it’s not just about skin. Various types of mud have been part of spa traditions around the world, thanks to their purported benefits to the body, both inside and out. In South Korea’s famously elaborate spas, called jjimjilbang, sauna rooms made of yellow mud are meant to help improve digestion. Some wellness seekers opt to sleep on traditional mats of solidified yellow mud.

But for the festival goers – who emerge covered head to toe in Boryeong’s most famous natural resource – it’s still all about fun. Walking away with silky, smooth skin is just a happy accident.