Is it possible to make a hole in your unity terrain ( I mean delte some triangles) so you can make a cave underneath it. I want to make a 3D model and set it under the terrain but can't find anything to make a hole in the terrain for the entrance.
Not really. You cannot delete any triangles or vertices. There is a trick you can do. This shader should do what you want. Place an object with this shader over the section of the terrain you want to be cut out.
One way to do it is use the paint height go to 20 hieght and 100 ocupicity and 100 brush size then cover the whole terrain at 20 so if you want to make a whole just put the hight at anything lower than 20.