"Use the zipline to reach the ground faster" is an objective in the quest Down in Amanaki Town in Far Cry 3.
Walkthrough[edit]
After disabling the Scrambling Device at the top of the Radio Tower, you need to get down, but not the same way you came
Note the Zipline on the side, and use it to get to the ground quickly
Notes[edit]
If you go down the regular way, the quest will still complete.
Quotes[edit]
Not getting back down that way (Jason)
