alt Hacker NewsFiber was all installed long after we knew locating it was important. I suspect it all had a tracer wire installed with it. Of course sometimes the tracer wire can break.
Before fiber a lot of things were put into the ground with no thought how you would find it again.
Indeed, tracer wire is usually bonded to a (sacrificial) magnesium anode, but it’s a crapshoot whether the tracer wire is still intact.
I sometimes hire directional boring and excavation contractors and if there’s any doubt as to where an electrical conduit or natural gas pipe is, I opt for the hydrovac truck to minimize risk.