Painting the underside of a terrain

Hi, beginner terrain user (and Unity in general) here...
I have a reduced scale mountain as a terrain and I'd like to be able to "enter" below the mountain. When I do so, I can of course see outside as the terrain is transparent (not rendered) when seen from below. Is there a way to paint/put material to the "back" side of a terrain? I thought about creating an "anti-terrain" with the opposite altitude and then rotate it 180 deg in x or z but found out terrains can't be rotated... I'd rather not abandon the terrain solution if possible...

 

Terrain is one sided mesh so you can probably make a hole but have to use a 3D mesh for any interior stuff.

 

Right. More than one sided it is an up-sided (y) mesh, if I understand correctly. No way of having a terrain facing down, correct?
In that case I was thinking about having somewhere else a terrain and a 180deg rotated camera so it looks like it is facing down would work, and moving from the up-world to the down-world using Terrain.activeTerrain.SampleHeight(transform.position).
Does it make sense or is there a better way of doing this (I don't necessarily need to be able to see both worlds at the same time, it is really an inside/vs/outside option)?
(writing this solution reminds me of Stranger Things...)

 

What do you actually plan to see when you're inside the mountain? That's usually a cave or something, which means a mesh and the problem of the terrain texture would resolve itself. You wouldn't work with that terrain anymore.

 

I'd like to see the "reverse" of the terrain as black or something like that. Right now my current solution was to export the terrain to a low quality mesh, put it there back, reverse the normals and eventually give it a material (currently the default one). Far from optimal. But modifying the terrain shader to have the terrain render normally and show black when the angle to the normal is > 90 deg looked way too much for my level of understanding of things (assuming this is possible and a correct way of doing things).

For the context, I have a real world map with a scale of 1/1000 and when I go "in" the moutain I want to see a 3D model of things that are inside the mountain (a tunnel and an underground laboratory). When I look "outside" I see the realistic map of the surrounding terrain, and when I "plunge" into the mountain I see the tunnel and the lab. Attaching 2 images to give an idea of the current solution (which isn't too bad, it's just than when you "enter" the mountain you briefly see that there are 2 meshes in there, the terrain and the mockup, if you look at the wrong angle). It is not supposed to be a "realistic" thing, entering in the mountain that way. The whole project plan is to do some science communication about underground science laboratories.

Outside view:



Inside view (currently default material for terrain, will probably put something maroonish in the future):