Neat stuff. I think the first works best too.
Thinking about what might make it read better, I tried 3 quick comps in PS using a section of your image.
The first was to see if my hunch that the backgrounds were too similar (in brightness and texture) to the interior spaces. I think this holds true as seen in image 1 and 2. Image 2 may not be what you are looking for, but brighter shades appear to come forward making your interiors appear to be hollowed out more. The 3rd image on the right is a quick paint over to dampen the shadow effect you had around the wall edges. It should not be constant everywhere, but should be less where the light is brightest on the walls (light reflection, etc.)
Click image for larger version.  Name: Water_Tunnels_1_2_cm.png  Views: 392  Size: 787.3 KB  ID: 32609Click image for larger version.  Name: Water_Tunnels_1_2_cm2.png  Views: 344  Size: 948.8 KB  ID: 32610Click image for larger version.  Name: Water_Tunnels_1_2_cm3.png  Views: 475  Size: 865.0 KB  ID: 32611

These are easy mods if you still have your source in layers, and moderate if you have to paint it out by hand...