From the Super Mario Wiki
Jeansowaty's Userbox Tower

It's MY USERPAGE!!! This.Is.MarioWiki Hell yeah!I like to play Mario games,and I have a fondness in finding worlds hidden in the games code in SMB.Currently,I'm trying to make a hack,which one would let you go to 8-5 on the SNES version.I am also a SMB hacker.I did a hack called Super Mario Bros - Jeansowaty Levels. I'm now creating a fangame called Mario Forever: The Lost Levels. Take a look at the MF TLL Gallery.

World 9 (SMB)(NES)

9-1 = Underwater 6-2.The game may freeze.Also,sometimes enemies randomly placed may appear like a glitched underwater koopa,or many cheep-cheeps flying trough the air.
9-2 = Underwater 1-4 or 6-4.This level,has only one enemy:The Underwater Koopa.Many of them appear in the level,mostly at the top.Completing this world will make you stop at the place where the princess would be,although nothing spawns there.Editing the level with SMB Utility will make it possible for you to proceed to 9-3.
9-3 = The hidden swimming part of 5-2 or 6-2.The enemies are the same:Bloopers and Cheep-Cheeps both white and red.Entering the pipe will result in getting again at the starting point.Editing with SMB Utility will help you to go to 9-4.
9-4 = A glitched,creepy level.When you enter it,it starts out with castle music anda '?'Block is seen at the upper part of the level.Sometimes,a Koopa may be seen falling out of the level.There is no time,so whatever you do,you can't complete it.SMB Utility can't help,as this level does not appear within it.

World 10 (World A) (SMB) (NES)

A-1 = The hidden cloud area of 3-1 and 6-2.Falling out of the level will result into going back to there where you have been.External hacking may eventually lead you to A-2.
A-2 = Same as 8-1.
A-3 = Another death level.This time, the header is set to Underground and it has some bricks and a cloud.No ground is present.Mario is set to walk when going inside the level, that's why it starts out with overworld music and you cannot use the GG code AEPLLG.