This is a scale map of the Korean Peninsula, which includes the entire South and North Korea, and also parts of Russia, China and Japan.
I made this using WorldPainter and DEMs of the peninsula.
The red line, made out of red stained clay, is the approximate location of the North Korean border, also known as "the Military Demarcation Line", which divides the peninsula into North and South Korea. This helps when roleplaying on a multiplayer server, where players can stay on the North or the South Korean side, or to create dedicated factions.
Update 3: Ocean Monuments no longer generate alongside the coast of Korea. Fixed it by changing the world type of the map.