Far cry instincts evolution map making help

How do I build underground tunnels? ahh, i cant figure it out lol

You can't create real underground tunnels because of how the game works. It works with height-maps and decolayers for the terrain, and unless you invent a multilayered heightmap, undergroundtunnels aren't possible. You would have to create 3d-models for them and import those, but I doubt that's possible here.

You can however dig trenches and cover them with wooden brdige objects/models and/or stone models.