Making terrain borders transparent.

Why? Well the terrain object can be used to make more than just mountains or islands. Sometimes you don't want the object border to be square. Bryce uses black to define the lowest point of altitude and white to define the highest. In the picture object, black will represent transparency as in the alpha channel in Adobe Photoshop, but that's a wee digression. Here's a before an after picture using the terrain for illustration purposes. The terrain is intentionally left 'floating' for the illustration.

Here's how to do it in the terrain editor, which is accessed by selecting the terrain(so that the wireframes are red, then choose the E for editing)

The small menu icon turns on the altitude 'slider' so that you can adjust the transparency.

The slider displays after clicking.

Drag the bottom end up and the terrain border changes color, indicating that the transparency is applied.