I have been working on a few special rendering modes for my Eden mapping software. The example shown here shows everything in grayscale, plus color(s) where there is a tunnel, cave, overhang, or anything inside a building (with a ceiling or roof).
The top is a normal map, the bottom is color-coded to show where the buildings and tunnels are. The colors are: red for deep, green for medium, and blue for close to the surface (plus buildings above the surface). The image is a part of "Six Flags Eden World BuilderV4" dated 03/30/2013.
Here are full maps of that world:
Normal map of "Six Flags Eden World BuilderV4" dated 03/30/2013
Xray map with buildings and tunnels color-coded by height or depth
(By the way, this is a fun world and the coasters are pretty big. The only bigger ones I've seen are in "Olympic Metropolis Port Angkor")