The GPS signal gets blocked with the 6 or 7 feet of dirt and concrete on top of it!
Years ago the company we worked for did a small city in Louisiana that wanted every single utility mapped, the streets, fireplugs, utility pole, and curb, driveway and so on. It was all put together in a GIS file that would let them click on a pole or sewer lid in the drawing, and bring up all the information possible about it. And the could turn on the image layer as well if they wanted to see what the ground looked like. We would love a few of those projects now.
We just did a smaller city and with the aerial photography and 3D map we made did the full planimetrics of every single object visible from the air. We don't have the info for is under the ground, but our files can be merged with whatever other data they had.
It is pretty cool to see a line drawing of an entire city. Click on any object with the info button and it will tell you the building address and who owns it. It can even be tied to the floor plan if they have that info available.