Where to put my OnMouseDown() to react to click on terrain?

I want to do something when I click on the terrain, and that something needs to use the coordinates of the RayCastHit. Usually if I want to tell if I have clicked something, I put OnMouseDown() eventhandler on the object's own script. Unfortunately Unity won't let me make a script called terrain, as it apparently already exists, so where should I put it instead?

 

Anything attached to the terrain GameObject should work

 

So you mean simply attach an extra script component to terrain, with a different class name, and put the mouse trap on there?

 

Yup