When you want to create complex poly like caves, you have only one option, 3D Softwares like 3DSMax, Maya, Gmax ...
You can do your caves by two ways :
- Poly : You will rumble a simple plane (create a generic plane with sectors and move the vertex to get a rumble effect.
- Texture : Create a simple tube (8 faces ?) and apply a photo realistic texture showing rumble rock cave.
I suggest solution two personally, its quick, and free a lot zones polycount, and most of the time, no one will notice its not rumbled walls (when texture is of good quality).
OpenZone, Dungeon Builder are great tools, but hit a limit when you want to do complex poly :/
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