The Utility Tunnels are the vast sewer system tunnels beneath Mainframe. An artificial system of pipe conduits run from each and every sector used to carry away and remove energy wastage from the city above. They also have electrical lines and various pipe-works running through. Pipelines that begin with connecting pipes from buildings and streets run to more levels of larger underground horizontal mains. At some point these terminate at treatment facilities within Pearson's Data Dump located in Giedi Prime. Vertical manhole pipes connect these mains to the surface.
This network was first seen when Dot finished gathering all P.I.D codes onto her organizer from citizens who were living in one of Megabyte's occupied sectors. This would effectively free them from his control. However the viral ABC forces had anticipated this and tried to arrest her. Cyrus quickly pointed out to Bob and Dot the Utility Tunnels would serve as an escape route outside the building. However they were relentlessly chased through the sewers by viral Binomes until arriving at a manhole. Cyrus was seemingly captured while Bob and Dot made it to the surface. (Identity Crisis, Part 1)
The tunnels were seen again when Megabyte wanted to get into the Mainframe Archives to retrieve the Gateway Command. Impossible through ordinary means, it was plausible to breach the protective security field from underneath. As this is second only to the System Operating Core, they setup an Energy Drill connected to the Leech Command that would drain the necessary energy from Game Cubes. With the collective energy from two Games they broke through. Enzo and Frisket had stumbled onto their plan but were captured before they could warn anyone else. (When Games Collide)