It was not that Song Shi was tunneling underground, but that he was using the Array Formation Power to teleport.
In the glamorous Imperial City, there were spiderweb-like Array Formations.
In the dark, many people used the Array Formation to teleport in and out, transmitting all kinds of information, transporting supplies, and even prisoners. There were also royal nobles who quickly shuttled through places with the Array Formation.
Originally, Song Shi did not have the authority, but he could now break the array to allow himself in. He was like a hacker who secretly controlled some authority. As long as he did not go overboard, he could even enter and exit the Imperial City without anyone noticing and head to some important nodes on the array.
For example, the Heavenly Prison.
