I heard there is a hidden/secret Minecraft-themed level of Borderlands 2. Where is it, what do I have to do to reach it, and what's the benefit from going there? Is it just an Easter egg or is it useful?
You can get Minecraft skin or head customization item and some other goodies there (like eridium).
Getting to Minecraft area:
To access the hidden Minecraft area, head to the Caustic Caverns after completing chapter 9. You will need to go to the North West corner of the map, called the "Guardian Ruins". You will see 2 tracks with minecarts on them. Go to the end of the track on the right, and jump onto the ledge. Then, jump down and enter the small cavern you will find there.You will find some dirt blocks that you can smash with a melee attack blocking your path.
Once they are broken, you can enter a larger cave containing the easter egg itself. Mine the back walls of the cave and a couple of the blocks will drop a Minecraft skin or head customization, enemies have a chance of dropping a Minecraft skin/head customization item too.
Video guide showing how to get to the area (spoilers ahead):