Ice torpedo

The Ice Torpedo is a weapon designed by the Mechanist for use in the invasion of the Fire Nation. They are first used in The Day of Black Sun, Part 1: The Invasion.

Overview
The Avatar and his allies were planning to invade the Fire Nation Capital on the Day of Black Sun, but the city was 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 Water Tribe Submarines to be armed with Ice Torpedoes, which would be used to blast through the barriers. The torpedoes are 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 the fuse to the blasting jelly remains lit but does not explode until the torpedo is fired. Launched through Waterbending, these torpedoes explode on impact, causing great damage to enemy positions.

These torpedoes were used to blast through the Great Gates of Azulon and the other harbor walls protecting the Fire Nation Capital, allowing the invasion force to land on the beachhead virtually unopposed.