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. This entity is still featured in the files of Counter-Strike: Condition Zero Deleted Scenes as weapon_zipline. There are also console commands indicating the zipline animation.
- ↑  Youtube - Easter Eggs, Bugs, Glitches - Counter-Strike: Condition Zero Deleted Scenes
- ↑  Youtube - Cut Content of CS - The Zipline - CCCS#17
- ↑ Game Informer - May 2003 Issue (2003) GameStop, Inc
- Zipline at Wikipedia