World 6-6 (New Super Mario Bros. Wii)
From the Super Mario Wiki
This level takes place in a dark underground cavern. There is a boat with a light that the player can move by rotating the Wii Remote. Various enemies, such as Piranha Plants, Cheep Cheeps, Swoops, and Fire Bros., make an appearance here. The water rises or lowers at some points of the level.
A secret exit exists in this level which can be found by entering a red pipe in the area with Fire Bros. The player enters an area with a Glow Block, along with a Swoop swarm. After that area is the Red Flagpole. This unlocks a Warp Cannon which takes the player to World 8. Players can run through this level with a Mini Mushroom by dashing across the water.