I heard Korea has natural hot springs?

Korea has great hot springs (온천), usually built into spa or jjimjilbang facilities rather than open-air pools. Well-known spots include Onyang and Asan (some of the oldest), Yuseong in Daejeon, Bugok in the south, and big spa resorts like Seoraksan Waterpia near Sokcho, while in Seoul you'll find hot-spring-fed spas in places like Dragon Hill and various large jjimjilbangs. Entry is cheap — often around 8,000–15,000 won — and the key etiquette is that bathing is gender-separated and fully nude, you must wash and shower thoroughly at the seated stations before entering the communal baths, and no swimwear is allowed in the bath area. Tattoos are generally tolerated these days, bring or rent a small towel, and a jjimjilbang also lets you use the mixed-gender sauna rooms in the provided clothes — it's one of my favorite parts of life here.