holes in terrains for unity 5

I'm not sure if I have this in the right place and I know it's been covered in other threads but I really would like to know how to create holes in a terrain for the purpose of tunnels and such.

I know it must be done via scripting and a Depth mask and I found a solution online but I'm running unity 5 and I don't think it's compatible, either that or I'm a complete idiot (most likely).

Also the only affordable asset (for me as a novice) isn't compatible with unity 5.

I am a complete noob at this and would love it if someone could give me some step by step instructions.

Al

 

I can confirm that the Terrain Hole System:
https://www.assetstore.unity3d.com/en/#!/content/8277
......does work on U5.

You'll loose the PBR terrain shader and don't start me on terrain shadows.

But you will have holes.

 

My Scene is getting pretty big so I've decided to go with Scene loading instead of trying to get the holes to work but thank you for you support.