Marius,
I think Michael is on the right track. By assigning a moderate to high level of transparency to the ore body, you'll be able to see the tunnels inside. What you might also want to do is the following:
1. Select the ore body mesh and set its transparency property medium to high (50% or more)
2. Set VSCURRENT to Realistic and choose object colors to what you want--dark gray and light gray come to mind, but it's your choice.
3. Use VISUALSTYLES to display the Visual Styles Manager, and create a new visual style that's starts with Realistic. Make it the current visual style.
4. Experiment with this new visual style--for example, set the Edge Settings to Isolines and turn on the display of Intersection Edges (to show where the tunnels intersect the ore body.
If you hate the results, delete the new visual style and create a new one based on Realistic. You'll want to leave the default visual styles unmolested so you can easily go back to a known setting.
Dieter
Dieter Schlaepfer
Principal Learning Experience Designer
Autodesk, Inc.
San Rafael, California