Ice Land

Ice Land (alternatively Iced Land, Winter Wonderland, the Ice World, or Ice Country ) is the sixth world in Super Mario Bros. 3. It is the largest world in the game, with ten levels, three Hammer Brothers, three fortresses, three Spade Panels, two Toad Houses, and a castle that has been taken over by an airship. As the world's name implies, it is a frozen wasteland covered with slippery floors and frozen lakes. Lemmy Koopa takes over this icy kingdom by stealing the royal magic wand and turning the king of Ice Land into a seal (a Monty Mole in the game's remakes). The map in Super Mario Advance 4: Super Mario Bros. 3 shows that Ice Land is north of Dark Land and east of Pipe Land.

The Adventures of Super Mario Bros. 3
In The Adventures of Super Mario Bros. 3, Ice Land, though never referred to as such, only appeared in the episode "Life's Ruff", where it had a different ruler named King Windbag. He bullied his servants and never said "please", but when Hip and Hop came, they stole his wand and turned him and Luigi into dogs and harassed them for a bit, but were eventually defeated and returned to Dark Land. The castle is depicted with igloos around it.

Super Mario World television series
An icy tundra, referred to as "Ice Land", appears in the first episode of the Super Mario World television series, "Fire Sale". Here, it is under the control of Kootie Pie, who had stolen Mama Fireplant to make it warmer. However, Ice Land began to melt because of the heat, and Mario, Luigi, Princess Toadstool, and Yoshi arrived to save both Mama Fireplant and Ice Land.

Trivia

 * The map theme from this world plays whenever Mario recieves anonymous e-mails in Paper Mario: The Thousand-Year Door. An arrangement, "Slowly, Slowly", also appears in Mario Party during the Pedal Power and Ghost Guess minigames. it also appears in super smash bros. ultimate as part of the song Fortress Boss - Super Mario Bros. 3 alongside the map theme for dark land and the titular fortress boss theme.
 * This is the only world of the game that is possible to go backwards from using a Whistle, as using it in the sixth world will allow one to return to the fifth. However, in Worlds 5 and 8, using it can cause one to return to the current world.