The Avatar and his allies were planning to invade the Fire Nation Capital on the Day of Black Sun, but the city 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 harbor walls 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, but the mechanist had to find a way to keep the fuse lit underwater until they could be used to fire on enemy positions. To fix this problem, the torpedoes are frozen in ice, so that the fuse to the blasting jelly remains lit but does not explode until after the torpedo is fired. Launched through waterbending, these torpedoes explode on impact, causing great damage to enemy targets.
- ↑ 1.0 1.1 "The Day of Black Sun, Part 1: The Invasion". Michael Dante DiMartino (writer) & Giancarlo Volpe (director). Avatar: The Last Airbender. Nickelodeon. November 30, 2007. No. 10, Book Three: Fire
- ↑ From older Avatar: The Last Airbender official site, originally on Nick.com (link). No longer updated.