Terrain Collision and Holes?

I know that this has been asked before, but there has never been a full guide / tutorial on how to make a hole in terrains. I wanted to ask, how do i actually make a hole in a terrain, i know that it is done through render queues and stuff but i cant actually find any specific indicator to how i can do it. basically, i have a default terrain, i want to then make a cave which is pretty much a hole and then make it so that the terrain collider disables, then allows the player to enter / leave the cave as they wish. I thought about this for quite a while the other day and figured a few problems with this method of putting holes in the terrain.

If i make a hole in the terrain and disable the collider when i enter it, wouldnt that mean that any NPC's on the map will also loose collision to the terrain and therefore fall under the map?

I thought of a small solution to this, but i need to incorporate the same method to actually accomplish it. I was thinking, what if i duplicated the first terrain, in the same position, made it invisible and gave it collision for MOBS only, then it would allow me to freely disable and enable the collision for the player easily.

So, could anyone please give me a push in the right direction to how i should hide / make terrain invisible and how i could possibly make a mob colliding terrain? ( Wouldnt it be through the use of tags?)

Anyway, thanks for reading this post, please help me ^^