The Lost Treasure
This can be started by opening a box containing an ECHO recorder in the Sawtooth Stilts. Kill all bandits you encounter until you collect all four map pieces. After finding all the pieces, you'll have to go to the Caustic Caverns to follow the clues.
The clues you're looking for are switches. Be careful as well since the enemies here are almost as the same level as you. Refer to the custom objective markers on the maps below to find the switches.
'Under the acid-soaked railway' - Oozing Discharge
'In the warehouse on the shore' - Infested warehouse
'In the Digger's Shadow' - Abandoned Mining Site
'Within Dahl's Bloody Sixth' - Deep Core Six
After activating all the switches, backtrack to the marked location and find the elevator that will take you to the upper floor of the facility. Fight your way through several spiderants nests and flying varkids until you reach the lever that will reveal a Dahl lootbox.