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The Muju Firefly Festival (무주반딧불축제) looks like a neat little festival coming up in the county of Muju, in the northern, more boring Jeolla province. The English-language website isn't very informative, but you can find a little more information on the Muju county page and you can get a few more pictures via a Naver search. There are a few more tourist destinations in Jeollabuk-do listed on Visit Korea.
Like the Butterfly Expo in Hampyeong, this one looks like it's geared toward kids, but I guess that goes without saying. That reminds me, I don't recall if I've ever seen lightening bugs over here. I don't recall if I've ever looked for them, but I know that catching them and holding them in an empty sunflower seeds jar was a summertime favorite among all the kids in my neighborhood.
I heard of this through Korea.net, a website I normally don't visit unless it's a slow news day and I need something to mock on my blog. Also on that site was a profile on themed museums on Jeju. Now, I know that Korea is a very conservative country *cough* and that foreigners do have very Victorian sensibilities, but how you gonna write a list of themed museums on Jeju without mentioning Loveland?
* Update: Came across a bus timetable for Muju, though it's nearly two years old. I have no idea how accurate it still is, but it will give you a rough idea and will let you know that unless you have a car you're pretty much spending the night in Muju.