World 9-1 is the first level of World 9 in Super Mario Bros.: The Lost Levels. It is primarily underwater, but uses graphics typically associated with overworld levels; because of this and the palette assigned to underwater levels in the original, many pipes and bushes appear with the pink coral's palette, and Bill Blasters are tinted the same green as the terrain. However, in Super Mario All-Stars, the palette oddities are fixed.
The player starts in a small area looking similar to the Warp Zones in earlier levels with a Warp Pipewith a Piranha Plant in it that leads to the underwater portion of the level. This is also the only playable level in the game to have the player enter the underwater portion willingly, as opposed to the player character automatically entering a Warp Pipe to enter the underwater or underground portion of the level.
The player is dropped into the water above a pipe with a Piranha Plant, followed by a Koopa Paratroopa. On top of a row of ? Blocks is a Bill Blaster, however it cannot fire any Bullet Billsunderwater. After some downward moving lifts above a current is another Koopa Paratroopa, followed by a group of pipes with Piranha Plants the player needs to swim through. After some more Koopa Paratroopas, the player will come across Lakitu, followed by a group of Koopa Troopas, Goombas, and more nullified Bill Blasters before coming to a Hammer Bro. After some Bloopers, Buzzy Beetles, and more Koopa Paratroopas is the Goal Pole.