Underworld

The Underworld, also known as the Underground, is a recurring location in the, usually serving as the home of Ganon and his minions.

The Legend of Zelda
In The Legend of Zelda, the Underworld can be accessed throughout various entrances scattered all over Hyrule that, upon entering, actually serve as the game's dungeons. In total, there are nine underworld entrances around the kingdom that Link must enter in order to retrieve a shard of the Triforce of Wisdom.

The Adventure of Link
Although the Underworld is not visited in The Adventure of Link, it is mentioned in the game's prologue where it says that Ganon's minions were calling up new monsters from the Underworld to work towards the revival of the evil wizard, implying that the Underworld is where the monsters actually originate.

Animated series
In the Zelda animated series, the Underworld is the home of Ganon and all of his underlings, and it is also where the Triforce of Power is located. The Underworld in the cartoon show seems to run underneath of all Hyrule since, instead of it being several separated labyrinths like in the original game, they all seem to connect. However, keeping with the original concept, it can still be accessed by several Underworld entrances hidden in the overworld. Some of these entrances are more accessible than others since some of them can only open or appear at Ganon's request. However, others are at simple view that Link and Princess Zelda are free to go through: in "The Moblins are Revolting", Link mentions that there is an Underworld entrance right in North Castle's dungeon.

Link and Princess Zelda continuously venture inside the Underworld via its many entrances throughout the show for various reasons, either to recover the Triforce of Wisdom from Ganon ("Cold Spells"), to obtain remaining the Triforce Shards that Vires stole ("Underworld Connections"), or to recover Link's body ("The Missing Link"). Ganon's lair, which is where the Evil Jar and the Triforce of Power are, is located in a mountain deep in the Underworld. It is also worth to note that Ganon is able to teleport anywhere in the Underworld, which he does to immediately find where Link and Zelda are: in the "Underworld Connections" episode, Zelda asks Link that he not use his sword while in the Underworld since otherwise Ganon will figure out where they are.