Because in all the books that mentioned the border, not a single one detailed how to cross it. Even those who had managed to pass through the border safely during its "peaceful periods" could never explain how they did it.
Generally speaking, they just walked up to the border and somehow got through. Then, once they'd finished their business, they'd step near the border again and, as if by magic, find themselves back on the main continent.
How they got across and how they returned—no one could figure out the underlying principles.
It was truly mysterious.
That was why the border was also called the "Mysterious Gate."
A mysterious gate so unfathomable that it left Eoryun with only one thought—he had absolutely no clue how to cross it, and it was driving him nuts!
Frustrated, Eoryun rubbed his face tiredly and forced a wry smile. "Don't tell me I have to shout something like, 'border, border, open up!' in front of it?"
