The subway system of the United Federation was very old. In 1863, Ingra established the world's first subway in its capital, Luodun Town, and within the next forty years, this mode of transportation had spread globally.
The subway in Opportunity City was first built in 1920, initially comprising only four lines. After three major renovations, four additional lines had been added during the process.
Building subways in this world was not a simple matter.
In 1914, the Dunweizhi Drilling Company of the United Federation discovered a door to the Great Void, enclosed in the rock layer 2000 meters underground during an oil well drilling project.
The drill bit had fallen straight in, and a monster in the form of a spiritual body followed the drill hole to the surface, resulting in over 200 worker casualties. This was also the first direct encounter with the Great Void shortly after the establishment of the Administration Bureau.