Well let's begin making a Dungeon Map.
1. Add a new layer directly under the grid. Fill it with White and name it "Dungeon".
2. Now take a black brush and start painting the dungeon. I turn the grid on so I can better orientate on the picture in my mind.
3. Blur the map a bit using Gaussian Blur (Filters - Blur - Gaussian Blur). the amount you blur it is highly dependent on how the walls are made. Are they natural? Take a high amount. Are they artifical? Take a low one. Mine are natural so I took about 100 pixels.
4. Add a new layer above the dungeon layer and fill it with Solid Noise Clouds. The values here will determine how random the walls will be. I used a detail of 9 and a
X- and Y-size of 10.
5. Add a new layer above the noise layer. Fill it with white and set its blending mode to "Lighten only". Now open the levels dialog (Colors - Levels) and slide down the white arrow at "Output Levels". Watch the dark area of the dungeon area appearing and stop anywhere you like. The dark area will define the walls.
6. Take the fuzzy select tool (the magic-wand-like-thing). Check "Sample merged" and set the Threshold to 0. Now select all the dark areas that are your walls.
7. Add a new white layer and fill the selected areas with black. Name it "Wall Mask" Turn the visibility of this layer off. You can now delete all the layers made after step 1 of this section. I keep the Dungeon layer for.. well... keeping sake.
8. Save the Selection to a Channel (Select - Save to Channel) and name it "Wall Mask"
9. Rightclick on the "Walls" layer and click on "Add Layer Mask". Check Channel and choose the Wall Mask. Repeat this with the "Wall Noise" and "Wall Bump" layers. I blurred each mask by 20 pixels and added a 5-10 pixel Noise Spread (Filters - Noise - Spread) because I think it looks better.
10. Add a new layer under the grid and name it "Chasm Map". Fill it with black and turn the opacity down so you can see the grid, the walls and the floor.
11. Take a white brush and start drawing the areas where your chasm will be (I really hope thats the right word for it...)