It is a very simple and very useful mod for the game. It helps you to find ladders and shafts that you can spawn in the game. You can also mark the stones that will spawn a ladder or shaft to a lower level.

If you can't see any markings on the screen, it simply means that you have bad luck and no stones will generate a ladder or shaft.

You can clear some stones until the RNG is in your favor then the marks will eventually appear.

You can also use the tilde key to force the appearance of the ladders in the Skull mines.