World 6-5 (New Super Mario Bros.)
From the Super Mario Wiki
This is an underwater level. Deep-Cheeps and Mega 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 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 Mega Deep-Cheeps and a Warp Pipe, Mario will reach the flagpole.