Absurdly Spacious Sewer: Not sewers, technically, but Imperial cities are known for their lengthy networks of service tunnels running under the streets, through which the actual pipes and cables that supply water, sewage & garbage removal, power, nanoslurry, bandwidth, and other utilities are routed, along with the pneumatic mail system and the tracks for delivery robots. (They would much rather dig the street up once – if a lot more – when they’re building it than have to dig it up again every time some individual system needs poking.) And you can, if you have the right master keys and don’t mind dodging the security systems, get just about anywhere you please via the service tunnels.
Well, except into people’s homes and other private buildings. It’s not like they don’t have basement doors down there just as secure as their outside doors are upstairs.
Also unlike the sewer passages, the service tunnels – after all, the sewer is a sealed pipe inside the service tunnel – are well-ventilated, generally quite clean, and kept reasonably free of vermin. Utility people have to work down there a fair bit, after all – chambers along the way hold various distributed bits of equipment.
(The storm drains and their sumps are further down, below all of these, the Underways, and the subway tunnels. Everything’s got to drain somewhere, right?)