Making Cave system in Unity 5

Hello,

I know there are many threads about this but none of them seems to work for me and I spent like 2 days already on that problem. Things that I've checked :

• http://wiki.unity3d.com/index.php/TerrainTransparency

• http://forum.unity3d.com/threads/hollowed-mountain.7791/

• http://www.smokymonkeys.com/kyrill/index.asp?direct=22

It's like I see the way to accomplish that but can't really glue all the pieces together. It seems that to make a "hole" in terrain we have to:

1. Place an Cube/Plane with depth-mask shader onto our terrain (mountainside e.g)

2. Add collider to it and script with OnTriggerEnter/Exit where we have to ignore collision between player and terrain

3. Put an object with our cave system (made in 3ds/blender) inside our mount-shaped terrain and camouflage the edge of entrance with other objects e.g rocks so we wont see any artifacts.

I don't want to use voxel terrain, instead of depth-mask I could use transparent terrain texture paint option.

Do You know any working example for version 5.3.5 or could You help me somehow with it?