World 6-5 (New Super Mario Bros.)
From the Super Mario Wiki
This is an underwater level. Deep Cheeps and Big Deep Cheeps make their only appearance in this level. Mario starts the level on a mountain, and must take a Warp Pipe to go underwater. Mario should continue moving to avoid the Deep Cheeps that home in on him, or otherwise hit them with a Fireball. The first Star Coin can be found near a Red Ring. Then, there are whirlpools that can suck Mario into a bottomless pit, causing the him to lose a life. The player must wait until the whirlpools shrink to pass them. The second Star Coin is located in a yellow pipe in between some whirlpools. When the player finds a whirlpool with blocks on the bottom of it, Mario should let himself get sucked into it, being taken into an area with the third Star Coin. After some Big Deep Cheeps and a Warp Pipe, Mario will reach the Flagpole.