(Slightly disturbing?) "The last train hasn't stopped running yet, Unnie."

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While the general consensus is to avoid not only underground areas but even semi-basements if possible, it is said that the number of people living underground is actually quite significant.
As evidenced in various videos, different types are living according to their own unique methods and cultures.
It is difficult to make contact in the first place, but because the gap is somewhat large to live together in harmony, conflicts often arise when they actually meet.
It is also said that delays in subway construction are mostly cases where negotiations drag on or problems arise during suppression.