ISPs or dark fiber providers have detailed maps aligning with road networks. If there’s a suspected fiber cut they’ll run an OTDR from the ODF box or the source photonic equipment which will show high reflection/ signal degradation where the break occurred to the nearest 100m dependant on test granularity.

> ISPs or dark fiber providers have detailed maps aligning with road networks.

Most of the time. I worked a summer for Virgin (UK) basically digging holes to run cables.

I'd say 1 in every 6-7 jobs we dug the hole and found no existing Virgin inf. in the ground. You'd have to come back the next day hopefully with better information to try again.

I believe they use devices that act like some kind of radar. They send a signal across the wire and measure how long it takes to bounce back. They can get a really good estimation of the distance between the ping and the cut.

But maybe others can explain how this works.

Edit: this website explains other methods very well https://www.electricaltechnology.org/2015/06/cable-faults-ho...

Also, fiber optics needs repeaters, so as a first rough estimate resource you can very quickly determine between which two repeaters the cable was broken.

Time-domain reflectometry