Capítulo 650: Chapter 348: The Disappeared Clockwork City_3

[The underground of Sky City is composed of high-density gemstone rocks which are hard and immune to magic power, and are rich in energy. Mining these layers, there's a chance of obtaining various types of renewable gemstones. Since the gemstone layer grows over time, if it is not mined periodically, it will continually constrict the space of the Underground City until it blocks all passages in the city.

Each level of the Underground City is like a large labyrinth, housing multiple nests of various underground creatures. (The monster nests have been unsealed with the birth of Sky City, and will continuously spawn monsters. It is imperative to dispatch personnel to clean them up regularly, to avoid overpopulation of demons in the Underground City.)]

Simply put, the Underground City is an experience gaining instance specially made for Fools. Not only can monsters be defeated here, you can also mine ores and gemstones along the way.