Minecart Mischief is started when you find an ECHO device in the Caustic Caverns. The ECHO device is near a small section of tracks between two doors. Pick it up and you'll be sent to push a minecart through the caverns.
- XP: 2333
- Money: 4 Eridium
- Gear: N/A
Finding this mission in the northern part of the Caustic Caverns is probably the toughest part. Once you've activated the mission, head further northwest and you'll see some train tracks that lead out over a gap. At the end of these tracks will be a minecart. The cart can't be destroyed so you only really have to worry about yourself.
Push the cart by running into the back of it and stop to kill enemies whenever you see them. Each time you come up to a set of locked doors, you'll have to push the cart as far as you can until it stops and then open the doors by using a valve to the left of the doors. The valve will be tucked into a little tunnel by the door on the left side.
At the end of the tracks, the cart will fall onto a conveyor belt. Shortly after, it will be processed along with its contents and it will spill Eridium onto the ground. You'll have to fend off some Varkids until this happens. Pick the Eridium up to finish the mission.