Like life under the oceans, more than half of The City is underground. We are comfortably oblivious to it except when the contents explode or leak or electrocute dogs. 
The City runs on the concealed works below the surface, and that invisible spectrum deserves a bit of celebration. 
Or simply some exposure to the light.

Cast glass manhole covers, lighted from below, are a reminder of the world below and can cast that spooky uplight in foggy or snowy weather. Cast glass is incredibly strong, and can be colored, pressed with decorations, frosted, clear or etched. Neighborhoods could have a local color or stores could extend their brand even further into the street.

The pattern of the manholes would suddenly make visible the seeming randomness of their locations, and help us understand what exactly is there. The next time something erupts we will, at least, remember that we live in just half of The City.