Go to Sanctuary Hole after Chapter 9. Jump off a cliff then go to Caustic Caverns. Run northwest to Guardian Ruins then go to the minecarts and turn right. Jump over the rectangular rocks then turn left to find Minecraft dirt. Break though it to reach Minecraft stone, cobblestone, and coal. Break though those to find Creepers, as well as blocks of gold, diamond, iridium and more. Kill the Badass Creeper to unlock Minecraft-themed skins.