From "Avatar: The Last Airbender: The Drill (#2.13)"
This drill is a feat of scientific ingenuity and raw, destructive power. Once it tunnels through the wall, our troops will storm their city. The Earth Kingdom will finally fall, and you can claim Ba Sing Se in the name of your father. Nothing can stop us.
Hmm, what about those muscley guys down there?
Please. The drill's metal shell is impervious to any Earthbending attack.
Oh, I'm sure it is, War Minister Ching, but just to be on the safe side... Mai and Ty Lee, take the Earthbenders out.