The Rat Granny

Once the underground world was built, it would become a playground for the rich. That meant the slums above would be torn down, leaving hundreds of thousands of poor people homeless. Where some people saw opportunity, others felt envy. It was all just a way for capitalists to make money, with no real purpose. So, just as construction of the underground world was nearing completion, someone released a poison gas bomb inside. The ventilation shaft above, more than a hundred meters wide, was blocked, and thousands died in a single night.

Afterward, the underground world became a notorious ghost market. Private investors were unwilling to shoulder the enormous cost of repairs, so the place was abandoned.