The ozone layer is a deep layer in the stratosphere, encircling the Earth, that has large amounts of ozone in it. The layer shields the entire Earth from much of the harmful ultraviolet radiation that comes from the sun. Interestingly, it is this ultraviolet radiation that forms the ozone in the first place. Ozone is a special form of oxygen, made up of three oxygen atoms rather than the usual two oxygen atoms.

The ozone was a focus in Highlander II: The Quickening