Andrew Zonenberg posted a Twitter thread a year or two ago where he fixed a missing PCB trace some layers down a PCB, with a stereo microscope, precision mill and very steady hands.

Edit: here's the thread. It's a 6 layer PCB with a short on L5 that needs to be fixed from the L1 side.

https://xcancel.com/azonenberg/status/1468825231225540611#m

10/10 read