Beneath city streets lies a web of fiber optics — hundreds of kilometers of glass threads carrying the internet. But accurate maps of this web are often lost or outdated. Finding a cable underground is like feeling for a wire inside a wall without knowing where it runs.

This new method turns the cable itself into an ear. Fiber optics already buried in the ground pick up the slightest ground vibrations. When a car drives down the street, the earth under its wheels compresses slightly, and the cable feels that deflection. Scientists take data from one end of the fiber and watch how the strain changes at each point. At the same time, they track the cars using cameras or GPS sensors on the vehicles themselves. Then an algorithm takes over: it finds the cable shape that makes the predicted vibrations best match the real ones.

The method has already been tested in simulations and on real sites. It doesn't require tearing up asphalt or digging trenches, just ordinary traffic and access to one end of the fiber. This approach could become a standard tool for inventorying underground utilities and even for monitoring the urban environment: vibrations can reveal road congestion or soil conditions.