The Underground City is found at depth of 300-303km. It has 4 buildings - The Forge, The Pyramid, The Oil Pump and The Armory. It introduces new resources (Oil and Gems) and a new game mechanic (Battles).
The Pyramid is located at the top, above the Oil Pump. The Pyramid introduces 15 new quests.
The Armory is located on the right. Here you can upgrade your Weapons.