The zipline is a piece of equipment cut from Counter-Strike: Condition Zero Deleted Scenes.
The only hint of its possible appearance is the presence of a viewmodel and worldmodel. Much like its real-life counter-part, it would enable a user propelled by gravity to traverse from the top to the bottom of the inclined cable.
The Zipline was probably meant to be used in skyscraper orientated missions, such as Rise Hard.
- Zipline at Wikipedia