World 5-B (New Super Mario Bros.)
From the Super Mario Wiki
This level is an icy level that contains a few Snailicorns. To begin this level, Mario needs to move while balancing himself on the slippery ice. Mario can get the first Star Coin by throwing a shell at it through a small gap. The second Star Coin can be obtained by pressing a P Switch and by jumping over gaps to get to it. Soon, the player should see a Snailicorn. Mario should go past it to find the third Star Coin. To get a high spot on the flagpole, Mario should Wall Jump to a higher platform.
To access the secret exit, the player must be Shell Mario. When Mario has to Wall Jump to a higher platform to get a higher spot on the flagpole, he should continue to Wall Jump even higher, going above the screen. Mario should go to the left to find a pipe with Brick Blocks in front of it. Shell Mario must break the blocks to enter the pipe and to find the Red Flagpole. This opens a pipe that takes Mario to World 5-Castle.