How did the Fire Nation get that huge thing thing over seas to the Earth Kingdom, and across the continent to Ba Sing Se? And furthermore, how was Aang, on top of the wall, the first one to spot it? Surely others must have seen it on it's way there who would have told the city about it. I do remember Suki saying something about the FN contolling the western lake in the serpent's pass, and that they were working on a secret project, but something tells me they didn't build that thing at sea.
I would imagine the drill was constructed in the Earth Kingdom at a nearby base, drawing upon colonial resources such as Yu Dao.
I suppose reconaissance isn't quite the Earth Kingdom's forte. Or they could have piloted the drill far enough away from loyal Earth Kingdom settlements so that no info could reach Ba Sing Se until it was too late.
suki stated they had control over the western lake. the serpants pass was a pass that separated the western and eastern lakes. essentially the territory around the wall on the western side was fire nation territory they built it nearby close enough otherwise it would have taken FOREVER to get it to the wall