Caustic Caverns map:


Go to the Caustic Caverns, (found through "Sanctuary Hole"

Go to the spot on the map above, marked with a red circle

You should see a wall of dirt that resembles Minecraft, melee the dirt to break it

Go through the dirt and you should be in a cave with creepers coming to you, they will drop the minecraft skin (called minecraft) and the minecraft head.

In the cave you can melee all of the "blocks", just like in Minecraft.