World 9 (New Super Mario Bros. Wii)

The World 9 from New Super Mario Bros. Wii is unlocked by defeating the final boss of World 8, and each of the eight levels in this world are unlocked by getting all of the Star Coins in the world of its level number.

9-8
The last stage of the game is a sky level. In it, the "floor" is made of several clouds that let Mario jump every time he falls on them. A little ahead, two ? Blocks are found and one of those contains a Propeller Mushroom, which will be really necessary throughout the time. A little more forward, the first Star Coin is found surrounded by Fuzzies and after this part, a Random Item Block is found surrounded by 5 Fuzzies, one being a mega. Below them, a ? Block from the right contains a Mini Mushroom that can help the player to access high places. After that, the second Star Coin is found alongside Fuzzies. Forward, King Bills begin to break down walls so Mario can continue his progress. The third Star Coin is found stuck between walls that a King Bill will destroy. Arriving at the end of stage, a last King Bill will appear near the flagpole where it could be destroyed if the player jump into the flagpole before being hit by it.

Trivia

 * It is similar to the Special World in Super Mario World.
 * The Rainbow Road theme from Mario Kart 64 and Mario Kart: Double Dash can be heard at some point in the World 9 theme.