From the Super Mario Wiki
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World B is the third secret world that was first featured in the game Super Mario Bros.: The Lost Levels. If the player completes this world, the player will gain access to go to World C.
||This is an overworld level with a few bottomless pits, a green springboard and a Coin Heaven.
||An underwater level which has several features including a long Fire Bar near the end.
||This level has very long pits which require the use of several platforms to be cleared.
||This level is a castle level which has a few lava pools in it. The second, third, and fourth pipes all go back to the first pipe (all the pipes contain red Piranha Plants). The last pipe leads to a Warp Zone to World D. At the end will be the Bowser Impostor, the axe, and the Toad.
The Lost Levels