Latitude
The thread that goes through Korea is the latitude line splitting up North and South Korea.
Why thread? Back in the old days globes were sewn together with cloth and thread, so there was a literal thread dividing the Koreas! Or you can just picture an actual, super long piece of thread going around the entire globe. That works too.
い
ぬき, よこいと
None
The only way you can see this latitude line is to ride an eagle (い) over it. The line isn't visible from a car, you're too close, and a plane is too high. You have to see it from an eagle's point of view.
Imagine being an eagle yourself, flying around the entire globe, following the latitude line that splits Korea down the middle.