Avatar Aang and his allies were planning to invade the Fire Nation Capital on the Day of Black Sun, but to reach the capital, they first had to make it through the Royal Plaza, a forum located in the harbor just below the capital. The plaza, however, was heavily defended by multiple walls and barriers that were seemingly impenetrable to any enemy fleet. To counter this issue, the mechanist, with help from Sokka, designed the waterbending-powered submarines to be armed with ice torpedoes. These torpedoes were used to blast through the three grates in the lower parts of the harbor wall protecting the Fire Nation Capital, allowing the invasion force to land on the beachhead virtually unopposed.
The torpedo itself consists of an ordinary warhead full of blasting jelly with a lit fuse inside. The torpedoes are then frozen in ice, so that the fuse to the blasting jelly remains lit even though it is fired underwater and so that they can be launched with waterbending. After the torpedoes have been launched they will travel forward until they make impact with the target, at which point they will explode, causing great damage to the target.
- ↑ 1.0 1.1 DiMartino, Michael Dante (writer) & Volpe, Giancarlo (director). (November 30, 2007). "The Day of Black Sun, Part 1: The Invasion". Avatar: The Last Airbender. Season 3. Episode 10. Nickelodeon.
- ↑ From older Avatar: The Last Airbender official site, originally on Nick.com (link). No longer updated.