January 2, 1949, Seongsan-eup
South Korean Army: 2nd Regiment based in Seongsan
Seobuk Young Men's Association: A right wing political group/militant group composed of men who had fled from North Korea due to persecution. Were vehemently anti-communist. (also sometimes referred to as the Northwest Young Men's Association as 'seobuk' means 'northwest' in Korean)
Minbodan: A right wing political group, but one that regularly included local Jeju residents (who were forced to join to show their anti-resistance group credentials) as a means to defend local villages
On January 2, 1949 soldiers from the 2nd Regiment along with members of the Seobuk Young Men's Association massacred 30 or so people from the village of Ojo-ri in an incident known as the 'Dynamite Incident'.
A few days earlier on December 29, 1948, soldiers summoned all of the villagers in Ojo-ri. Anyone who had dynamite in their possession, knew how to use the dynamite, or were simply late to gather were taken to the base or imprisoned in a potato storage warehouse.
These people were then executed at Woomutgaedongsan (우뭇개동산) in what was the single largest massacre during 4.3 in Seongsan. You can find Woomutgaedongsan right at the base of the famous Seongsan Sunrise Peak (성산일출봉) just as you enter the area past the ticket check booth.
References
Jeju Dark Tours - '우뭇개동산' (readable here in Korean)
No Cut News - '4.3학살 흔적 지워지는 제 성산일출봉' (readable here in Korean)
Photo courtesy of No Cut News (Ko Sanghyeon) (nocutnews.co.kr)