World Star (Super Mario 3D World)
From the Super Mario Wiki
World Star (called Star World on Miiverse) is the first special world and the ninth world in Super Mario 3D World. It takes place above the Sprixie Kingdom, and can only be accessed by completing World Bowser, and taking the Rocket in World 1 which was built by the Sprixie Princesses. It contains nine standard courses, a Captain Toad's Adventures course, and a Sprixie House. Upon completing World Star-2, Rosalina becomes available as a playable character. After completing Cosmic Cannon Cluster and collecting the necessary amount of Green Stars, stamps, and gold flags, the rocket to World Mushroom will be available. It appears to be made of a water-like substance, due to the ripples and splashes made when walked on.