Avatar: The Last Airbender Season 2 Episode 13 : The Drill

Having successfully crossed the Serpent's Pass, Aang is determined to go to Ba Sing Se where he hopes to find his lost bison, Appa. However, he discovers a Fire Nation drill heading straight for Ba Sing Se, intent on penetrating the wall. Aang and the group succeed in stopping and destroying the drill from the inside. Meanwhile, Jet wishes to recruit Zuko for his Freedom Fighters, but learns that Zuko and Iroh are Firebenders.