what level is the underground city in mr mine

According to the Mr. Mine Wiki, the Underground City is found at a depth of 300-303km. It has four buildings: The Forge, The Pyramid, The Oil Pump, and The Armory. The Oil Pump and Gem Forge are unlocked at this level, as well as weapons and monsters. The Forge is a medium house on the left with three big gems on top of it, where players can craft five different types of gems using certain resources, time, and slots. The Armory is a mighty statue located on the right of the Underground City, where players can unlock and upgrade weapons to defeat bosses.