Below Utropiia there is another city, one of pipes, boilers, generators and machinery.

This vast and intricate maze of rooms, corridors, chambers, and conduits provides Utropiia with clean water, electricity, steam and heat, and powers the subway system and the city's elevator system of platforms that carry Utropiians from one level of their city to another. 

This hidden underground system extends as deep into the ground as Utropiia skycrapers are high and requires an immense workforce to maintain. It is so colossal that reaching the deepest and oldest parts of it can take days of travel, and that some workers servicing it can spend months underground at a time. 

Houses have even been built to accommodate these workers on their long underground missions. These living quarters are built on tracks and move through the caverns under Utropiia, allowing the engineers to travel in some comfort through the Utropiian underground as they complete their inspection tours.