It seems that a bandit has hidden a treasure that he got from Old Haven somewhere, splitting a map he made into four pieces that he gave to some bandits. Go into the large area highlighted on your map and simply kill the bandits there until they drop the four necessary map pieces. These pieces are completely random, so keep killing and if you happen to run out of bandits, save and quit to make them respawn.

With that done, now comes the hard part, as you have to follow the clues to find switches in the Caustic Caverns. You’ll be given a big general area on the map and your goal is to find four switches in it. This can prove very difficult, as you are only given vague clues as to their locations, but luckily for you, the following images will show off each switch.

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Under the acid-soaked railway

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In the warehouse on the shore

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In the digger’s shadow

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Within Dahl’s bloody sixth

Once you’ve hit all four switches, you will find a big elevator waiting for you near the Nether Hive and Abandoned Mining Site areas. Take this up to reach the Varkid Ramparts area, where despite the name, you’ll encounter a lot more Spiderants. Once you reach the end of this area, the mission will update and you’ll find a switch, where you can turn in the mission.