I live in Kyiv, Ukraine. I know a guy here who's hobby is exploring the old underground tunnels – sewage, overflow channels, catacombs. He's making a map of these because there's currently no official complete map. He often finds places there in a very alarming state – like, about to collapse and create a huge hole in the ground – and reports this to muni service so they can fix it and prevent a disaster.
Check with the city first - in the US they often have a management program due to the need to inspect manholes. Depending upon how advanced they are they may even have georeferenced photos and age information.
One of my still-on-the-shelf projects is to photograph, geo-locate and categorize all the manholes from my city that have been installed prior to WW2 in order to document how the city evolved over time (surprisingly enough my city still has manholes from the early 1900s), because a city with no working sewage system cannot be called a "city".