How do I make islands with the terrain editor?

I need to make detailed islands with many trees and plants, and I want to do this in the terrain editor. The catch is: I want the islands to be floating in space, with nothing in between them. How can I do this? is it even possible in the terrain editor?

if you want to be able to see the terrain from bottom then you need to use a 3D application to model the terrain. adding trees and some other details is possible with unity built-in terrain editor.