The drill was a secret Fire Nation project. When they premiered the two episodes as a two-parter during the original run of the show, they took the first episode to illustrate the Gaang’s troubles of getting to Ba Sing Se without Appa while hinting at the classified Fire Nation scheme that had been being conducted right outside Ba Sing Se for a presumably long time, then getting to the pay-off by revealing that the “Secret(s?) of the Fire Nation” was the massive mechanical drill designed to penetrate the walls and give the Fire Nation Army an in through which to invade and conquer the city. It’s not the best name tho, I’ll give you that
