Ok, so in Borderlands 2, when you unlock access to the Eridium Blight, you can go back to Claptrap's Place to find an item called Geary's Indestructible Gear in the fireplace. Supposedly, if you take it, then go to Eridium blight on foot without fast-travel, vehicles, or dying, you can bring it to Geary at the volcano, he'll hit you to take it from you, then jump into the lava and cause rare loot chests to appear. My friend and I did this last night, but when Geary attacked us he didn't do anything unusual. He just attacked us. Did we do something wrong, or did we not understand the challenge correctly?
From the Wiki:
This Easter Egg can be completed in co-op mode; the other players who didn't pick up the gear must stay far away from Geary, preferably at the base of the volcano. Because of this, completion in co-op can prove to be difficult for some players.