Subway Driver

"You have to remember that Qingcheng is a place that our Liu family will protect for generations!" "Grandpa, why is this?" "Because, this is a rule handed down from our ancestors, and we are all the guardians of this city!" "Grandpa, why on earth is that?" "Because, under the feet of Qingcheng, there is another city..." To this day, I still don't understand what my grandfather meant by another city...