The city has an intricate network of underground routes. For examples sewers run everywhere. These tunnels can be used to route fibre-optic cable because it is flexible and immune to water (but unfortunately not to rats who can chew through them!)

Other underground facilities that have been used to pass cables through are existing rail tunnels or telecommunication conduits.

Have a look at some news stories where this is currently happening:

Broadband comes up from the sewer

High Tech underground

Cities go underground for fibre optics