At the Sanctuary Hole after Chapter 9, jump off a cliff and go to Caustic Caverns. Run northwest to Guardian Ruins then go to the minecarts and turn right. Jump over the rectangular rocks and turn left to find Minecraft dirt. Break though it to reach Minecraft stone, cobblestone, and coal. Break though those to find Creepers along with blocks of gold, diamond, iridium and more. Kill the Badass Creeper to unlock Minecraft-themed skins (Steve Head, Steve's Clothes etc.).