This is a side quest in which you have to locate considerable deposits of Elexit. Reaching it requires dealing with some cannibals and completing an unusual climb.
How to unlock the quest: Talk to Cindy in The Fort.
Cindy is in a club in the “fun” district of The Fort (the base of the Outlaws). During a conversation with her, you can give her 100 Elexit in exchange for a piece of valuable information. She will let you know a location of a thief, who’s sitting on substantial Elexit deposits.
You have to set out to the south, towards the lighthouse near the edge of the map. You can reach the lighthouse via the drawbridge. Be careful, for a group of cannibals led by Hopkins is in the vicinity of the lighthouse – deal with them (you may finish at the same time another quest – History Lesson) or use the jetpack to try and bypass them (if you want to fight then try to lure them out one by one).
After you deal with the cannibals, you’ve got to get on top of the lighthouse. Use your jetpack to reach the wooden balconies (picture 1). After reaching each balcony, you might want to save your game and wait for the jet pack bar to replenish. Get to the top of the lighhouse (400XP). You will find Large Bags of Elexit (picture 2). Take them all.