World 1 (Super Mario Bros.: The Lost Levels)

World 1 is the first world found in Super Mario Bros.: The Lost Levels. The world is well-known for bearing many similarities to the original game's, but the sprites had been advanced and the levels were much harder. This was the first world in any game to introduce the Poison Mushroom.

World 1-1
The player starts out next to the castle on a rocky pathway. As the player moves forward, he/she will encounter a Koopa Paratroopa. The player must avoid this enemy, and then stand under a layer of Brick Blocks. If the player hits the brick that's second-to-the-left, a Mushroom will appear. Then, the player must avoid the two Piranha Plants and jump over a green Pipe. Over the tree next to the Pipe, there is an invisible 1-Up Block that the player can get (optional).

After jumping over a hole, the player will come across a swarm of Koopa Troopas. The player must avoid these and jump over another two Pipes (one of which contained a Piranha Plant, and the other lead to a coin room). At the last ? block of the level, the player can find a power-up.

Now, the player will see a staircase with the Flagpole. Slide down it to complete the level.

World 1-2
This is the first underground level of the game. There is a small cutscene at the beginning of the level showing Mario or Luigi walking into a pipe in the overworld.

In this level, there are Goombas, Piranha Plants, and Koopa Troopas the player will have to avoid, and a large canyon where the player must quickly jump over the platforms to cross. Now the player will have to cross pipes infested with Piranha Plants, and some Koopa Troopas.

Here, the player can either go to World 1-3, go up a vine to the World 3-1 Warp Zone, go above the exit to 1-3 to a World 4-1 Warp Zone down the first pipe, or continue on to a World 2-1 Warp Zone. When going down the pipe to the world 4-1 Warp Zone, if the player loses a life, he/she will start at the beginning of the level instead of at the midway point.

World 1-3
World 1-3 takes place in the heights, or Mushrooms in the overworld. The player must jump over large gaps to get from Mushroom to mushroom. There are also many enemies to avoid, most notably overworld Bloopers. The player must also cross platforms which will drop if stepped on. Since there is no staircase at the end (it is replaced by a platform), the player must jump on the platform to get to the flag. Clearing this level gives the player access to World 1-4.

World 1-4
World 1-4 is the first castle level in the game. It is very short, and the player needs only to jump over nine holes while avoiding the nine firebars, and then the player will reach the first False Bowser in the game. If the player is in his/her Fire form, he/she could hit the false Bowser five times with fireballs in order to reveal a Goomba in the costume, and then the false Bowser would fall into the lava, meeting its demise. The player could also touch the Ax for the bridge to collapse.

After defeating the False Bowser, the player will walk up to a Mushroom Retainer, who will say "Thank You Mario! But Our Princess Is In Another Castle!" The player will now have access to World 2-1.