1998 Yosu submersible incident
The 1998 Yosu submersible incident was a naval skirmish that occurred off of the southern coast of South Korea between 17 and 18 December 1998. On the evening of 17 December, a South Korean observation post sighted a North Korean semi-submersible naval vessel in the vicinity of the city of Yosu.
Source: Wikipedia — 1998 Yosu submersible incident (CC BY-SA 4.0)