November 17, 1948, Seongsan-eup
Seobuk Young Men's Association (서북청년단) - A right wing political/militant group that was composed of men who had fled North Korea due to persecution. They were vehemently anti-communist. (Are sometimes referred to as the 'Northwest Young Men's Association' as the word 'seobuk' means 'northwest' in Korean)
Residents from Seongsan (성산) and Gujwa (구좌) regions
Teojinmok (터진목) is a small land area that would appear and disappear depending on the tides. During the April 4.3 Incident it was used as an execution location by the Seobuk Young Men's Association who were stationed in the Seongsan region. Initially, the Seobuk Young Men's Association was brought over to help break up strikers and assist the police, but as time went on they gained more and more power. Eventually they became a force with more power than the police as they had virtually no oversight and could do whatever they wanted.
Near Teojinmok, there was a large warehouse where the locals would store potatoes. The Seobuk Young Men's Association used this warehouse to jail people they brought in from the Seongsan and Gujwa regions. Over the course of five occasions: November 17, 1948, January 9, 13, and February 1, 15, 1949, 65 people were executed here.
References
Jeju Dark Tours - 터진목 (readable here in Korean)
Jeju 4.3 Archives - 터진목 (readable here in Korean)
Photo courtesy of Jeju 4.3 Archives (43archives.or.kr)