The Underground City is found at depth of 300-303. 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 Underground City
The Forge is located on the left. Here you can forge Gems, a resource that is needed for upgrading Weapons. The Forge is the only source for obtaining Gems.
The Pyramid is located at the top, above the Oil Pump. The Pyramid introduces 15 new quests.
The Oil Pump is located in the middle, below the Pyramid. Here is your oil stored. It slowly recharges until it reaches available capacity. Upgrading the Pump increases its max capacity.
The Armory is located on the right. Here you can upgrade your Weapons.