Flat terrain problems in new terrain syetem!

I can’t create a lower path in new unity terrain system.So sad.
I’ve tried so many ways, still can’ t figure that out.

1.In set height panel: Can’t set height to a negative number.
2.In stamp terrain :Can’t stamp a negative pattern.

So the only way for me is to raise the whole terrain while leave a narrow path?
It should have much easier way.
@Bunny83 @Eric5h5

Figure half of that problem out, in Set Height panel press Ctrl can sample the height at the cursor position.(Though in Official maunal it says “Alternatively, press Shift and click on the Terrain to sample the height at the cursor position, similar to how you would use the Eyedropper tool in an image editor.”.Looks like thay made a mistake.
But sadly ,it can’t sample a negative number.