A character can work out the gradient of a tunnel, and hence how much he/she is getting deeper or closer to the surface, by making a dice roll using his Mining (including Navigation underground/judging gradient of tunnels specialisms) + PE.
The difficulty is 10 to get the gradient of the tunnel absolutely accurately. If a character fails the dice roll, he/she will get his/her orientation wrong by up to 5 degrees per point by which he/she fails the dice roll - the referee should give the player misleading information accordingly.