Use a modelling program such as 3ds Max, Maya or blender to create a model which you can import into unity.
Blender is free so it's what I'd recommend.
answered Feb 05 '11 at 03:16 AM
see this http://answers.unity3d.com/questions/46969/caves-and-underground-bunkers.html#answer-141545 and look at "winamp monkey" in youtube to see tunnels generated with metaballs, there are metaballs script in unity community, else i was going to make some tunnels with xenodream or some generative code... gen code is EASY to deform a long cylinder and bend it around etc, add noise, apply diameter functions... Then... flip the normals inside the cylinder so have textures on the inside. but it is more difficult to make caves with multiple branches and forks etc.
answered Jul 10 '11 at 02:46 AM