How can I make small circular hole on the terrain?

#1 by making a 3d mesh and placing it in a non-circular hole

#2 @Chris,
Well, there is a reason that I want to make a circular hole instead of placing a mesh with a hole. Usually most of the greens have different textures. That means we have to make different hole meshes or at least textures for them. If making those meshes go well with the terrain is simple, I don't need to worry about it. The truth is that it is not.

So, is there someone?

#3 I would model the entire greens with the hole, and bring it in as collada. The only holes you're going to get out of the terrain will be rather large rectangles with little hope of shape optimization.

Depending on the style of your game, you may be able to ignore all that modeling stuff, create a simple 2D hole decal.

#4 @Brian,

I think creating 2D hole decal is a good start. If I don't render the terrain under the hole decal, and make the hole in the decal transparent, it will be possible to see the holecup through it. Thank you.

At this moment, I'm not sure which source code that I have to change.
So, please someone with deep knowledge in engine code shed some light on me.