The "middle", as in you want to tap a trace and not a pin on a chip? You really can't without hacking at the board somehow, there is a plastic resist layer on

The "middle", as in you want to tap a trace and not a pin on a chip? You really can't without hacking at the board somehow, there is a plastic resist layer on all the traces. They're usually visible through it but without scratching a hole you can't get to it. And then of course the signals you actually care about are just as likely to be in middle layers you can't reach without destroying something above/below it.