In exception of pistons.
Quote from xWoodie »Not to be negative, but I doubt it is a new biome. If Notch were to add a new biome, he would have hinted it to cause excitement. It would make a great first home though!
Quote from viper23 »No, this is just a glitch, or there was already an opening. Grass will spread, and it's pretty lucky if the floor is all grass, but I doubt it's a cave biome. Cool find though.