New Super Mario Bros. Wii: 1-4 -- Sleeping with the Cheep-Cheeps

This level takes place underwater, with some nice soothing music. From New Super Mario Bros. (DS), I know that some secrets lie beneath the main level of the game, in what normally is a void. I tried to check out the first gap, which turned out to be a void, and I died. I won't be trying that trick just yet for a while, so I can have lives for reaching the next save point.