The story
June 8, 1970: Subway construction headquarters
April 1971: Line 1 (Cheongnyangni Station - Seoul Station) construction started and opened
October 1974: Gunja Depot construction
March 9, 1978: Line 2 Gangnam Section construction started
February 29, 1980: Line 3 and 4 construction started
May 22, 1984: Line 2 Circle Line opened
October 18, 1985: Line 3 and 4 opened
July 13, 1990: Line 3 (Gupabal Station - Jichuk Station) opened
Read the full text
May 22, 1992: Line 2 Sinjeong Branch (Sindorim Station - Yangcheon-gu Office Station) opened
April 21, 1993: Line 4 (Sanggye Station - Danggogae Station) opened
October 10, 1993: Line 3 (Yangjae Station - Suseo Station) opened
April 1, 1994: Line 4 (Sadang Station - Namtaeryeong Station) opened
March 20, 1996, Line 2 Sinjeong Branch (Sinjeongnegeori Station - Kkachisan Station) opened
October 20, 2005: Line 2 Yongdu Station opened
December 21, 2005: Line 1 Dongmyo Station opened
February 18, 2010: Line 3 (Suseo Station - Ogeum Station) opened
March 28, 2015: Line 9 (Eonju Station - Sports Complex Station) opened
May 31, 2017: Merged with Seoul Metropolitan Rapid Transit Corporation & Seoul Metro
From Wikipedia’s article on Seoul Metro Corporation, shared under CC BY-SA 4.0.4