So, I started a new world the other day and in the center of the map there was a cobblestone obelisk leading down to a cave. Inside the cave there was no diamonds/lapiz/redstone/iron, only coal, plus the entirety of the cave was lit by torches. Judging by the size of it, it was the 'main' cave of the seed. Can anyone explain how this happened? I was playing offline and by myself so nobody joined. Seed: Xbox 360. Cool seed otherwise.