Okay so you know how you have chunks and they map to 4 levels of biomes? Logically you'd go hmmmm, let's just map 4 Y levels to one biome level. Except there aren't 16 chunk heights, there are about 17 if you count Y=16 (which can contain data), but there's also -1 technically but I've never seen this have data in an NBT structure I don't think. Anyway, let's just hope we're doing the right thing by giving Y=16 the same biome as Y=15. If someone wants to check whether that is actually correct please feel free to, but for now it's better than crashing at the very least. Would be nice if we had a spec for these kinds of things. Fixes #1685.