Ice Cavern (Ocarina of Time)

The Ice Cavern is a mini-dungeon appearing in Ocarina of Time. This dungeon acts as a prerequisite to the Water Temple for the Iron Boots.

Entrance to the Cavern
It's a frozen cave behind Zora's Fountain, though that isn't its original condition until later in the game; after his seven year sleep, Link finds Zora's Domain frozen and the Zoras encased in ice. King Zora is encased in a mysterious Red Ice, as is the entrance to the Zora's Shop. Lord Jabu-Jabu is missing from the fountain and is replaced with a path of ice floes leading to a previously inaccessible cavern, the Ice Cavern.

Themes and Navigation
With the Gerudo's Training Ground, the Ice Cavern is the smallest in the entire game: it only has one floor and, unique among all the other dungeons in the Zelda series, each rooms (with the exception of the individual mini-boss room) are connected by hallways instead of doors, giving the illusion of one extremely large room divided into rooms and hallways. It is ice-themed, and inside Link finds Blue Fire, an unnaturally occurring fire only found within the cavern and the Water Barrier in Ganon's Castle (although it can also be bought in the Medicine Shop for an exceptionally high price). Blue Fire melts Red Ice, and as such, Link uses Bottles to carry the fire and melt the Red Ice that is found throughout the cavern to unblock certain paths and rooms..

Link receives the Iron Boots in the Cavern after defeating a White Wolfos. Sheik appears to tell the young hero about the Zoras frozen beneath Zora's domain as well as telling Link he saved Princess Ruto, who headed to the Water Temple underneath Lake Hylia. Afterwards, Sheik teaches Link the Serenade of Water, the warping song of the Water Temple, before disappearing again.

Master Quest
In the Master Quest version of the game, this dungeon is actually easier. The spinning scythe is gone, and there are very few Freezards and Keese. Instead of the ice block puzzle, the Song of Time can be played to make a block appear, which gives access to the Blue Fire. Some switches need to be activated to make some platforms disappear. However, they are very well hidden. One switch is camouflaged between some diamonds, another is behind a stalactite and the last one's tip is protruding from the floor. Because there is Red Ice blocking several ways, the first time Blue Fire is seen is where the Piece of Heart originally was (it has been moved to the other room with Blue Fire). New features include boulders rolling in connecting rooms and two White Wolfos throughout the dungeon. The miniboss is now a Stalfos.

Ice Temple Theory
In the Beta version of Ocarina Of Time, the Water Temple was the Ice Temple. Some fans believe that after the temple was changed in the final version of the game, parts of the old Ice Temple were reused, and placed in Zora's Domain as the Ice Cavern and the temple's original theme music was reused as the Ice Cavern's theme. It is also possible that both the Ice Cavern and the Ice Temple were present in the beta, as all the other mini dungeons match their succeeding temple, such as the Kakariko Well and the Shadow Temple being shadow-themed.

Snow Peak Theory
Some fans speculate that the Ice Cavern is a section of Snowpeak, which is seen in Twilight Princess. Snowpeak is located just next to Zora's Domain, which is where the Ice Cavern is.

Trivia

 * In Twilight Princess, there is a cavern similar to the Ice Cavern, located in the northernmost portion of Hyrule Field; its puzzle feature is also similar: Link has to move blocks to make them slide through the frozen floor in order to press switches that open new doors, leading Link to a chest with a Piece of Heart.
 * This dungeon is the first to include a Piece of Heart within. This practice would recur in later games, appearing in some mini-dungeons in Majora's Mask, in one case in The Wind Waker, more frequently in The Minish Cap, systematically (with two in every dungeon except the final dungeon) in Twilight Princess, and occasionally in Skyward Sword.
 * This Mini-Dungeon is similar to and may have been inspiration for the Cavern on Ice Ring Isle from The Wind Waker as both are accessed by jumping across Ice floes, both center around ice, and both hold the Iron Boots. They both also have an easier way to get back to the entrance with the Iron Boots.
 * The Ice Cavern is the only dungeon in Ocarina of Time that occupies a single floor.