Iceberg spike

Iceberg Spikes are artificial segments of frozen water created by Waterbenders for primary defense against intruders. They are mostly seen scattered around the vicinity of the Northern Water Tribe, in the ocean surrounding the Water Tribe stronghold. They resemble large, jagged icebergs, and are strong enough to tear through metal. Coupled with the vicious currents in iceberg fields, they are able to sink even the largest of ships.

Northern Water Tribe
Iceberg Spikes serve primarily as defensive structures to fend off intrudes, most specifically Fire Nation ships. Specific spikes are placed strategically in the waters surrounding the Northern Water Tribe, and a number of others can be easily created through Waterbending. Iceberg Spikes, when coupled with strong ocean currents, can easily sink incoming ships.

In addition to their defensive purpose, Iceberg Spikes also serve as ambush points. Water Tribe Warriors can dock behind the spikes, and raid approaching vessels successfully. The segments are also advantageous for Waterbenders, who are able to manipulate the spikes from nearby boats, providing them the upper hand in an open sea battle.

During the Siege of the North, Waterbenders created an enlarged Iceberg Spike to immobilize a Fire Nation Navy ship, suspending it above water and ripping through its exterior with relative ease.

Southern Water Tribe
Several Waterbenders from the southern faction utilize Iceberg Spikes in battle, as seen when they defend their city against Fire Nation invaders during the Southern Water Tribe Raids.

As a group, Hama and several other Waterbenders used Waterbending to suspend a Fire Nation vessel above the ice, through the use of an Iceberg Spike. The ship remained in the western regions of the South Pole, and was commonly known as The Shipwreck, a reminder of the Fire Nation attacks that occurred during the early stages of The War.

In Battle
Renditions of the Iceberg Spike are incorporated by Waterbenders in combat. While under the tutelage of Master Pakku, Katara defeated pupil Sangok by immobilizing him through the use of a Waterbending move similar in appearance to Iceberg Spikes. With ease, Katara redirected Sangok's attack, accumulating the water he produced into a large wave, before lifting him up and freezing the water, ending the duel.

While searching for Aang in the frozen tundras of the North Pole, Katara utilized this move yet again, though in a much grander scale due to the presence of a full moon. With ease, she lifted the snow underneath Zuko, and raised it up high above the ground before ruthlessly slamming it down, debilitating Zuko successfully.

Trivia

 * Katara uses a rendition of this move against pupil Sangok, while under the tutelage of Master Pakku. Katara also uses ice spikes in battle, a move requiring a similar form. She used this against Combustion Man during his attack on the Western Air Temple, and against Yon Rha, who she wanted to punish for the death of her mother.