Crater

Occurance

Craters are a rare biome generation where there is a huge gaping hole it the ground that can lead to near-bedrock areas.

Size

The current possible sizes are unknown, but SGCBarbarian on Youtube has posted two videos of Natural Spawning Craters.

http://www.youtube.com/watch?NR=1&feature=endscreen&v=u2XJKoqMiWs