World 1 in Super Mario Bros. is the first world in the game (there are eight in total). Naturally the four stages featured in World 1 are considered the easiest of the bunch. The four stages span four different environments including above the ground, the underground, on the tops of trees, and in Bowser's Castle.
World 1-1 starts out on a brick path. As Mario moves forward, a Goomba appears and starts walking toward Mario. The red-clad plumber also reaches the first Question Block of the game, and afterward the six block triangle formation of ? Blocks and regular brick blocks (with a Super Mushroom inside the left block). A set of Pipes await Mario with Goombas located in between the plumbing. The final pipe among the set leads to a bonus area of coins and can be entered to bypass much of the level. After that, assuming the pipe was not taken, there is a gap followed by another Question Block containing an item, possibly a Super Mushroom or Fire Flower, depending on Mario's health. Additionally, Goombas start falling down from a long row of blocks above.
Afterward, a block down below contains ten Coins that Mario can retrieve by Jumping repeatedly underneath the brick. Then there is a notable ? Block triangle formation, which contains no regular blocks. The top block contains a Fire Flower (or a Super Mushroom if Mario is small). Upcoming are some more Brick and ? Blocks, followed by a pyramid-like set of unbreakable blocks with a gap in the middle. After that is another set of unbreakable blocks, this time with a gap in the middle that Mario can fall into and get killed. Just following is another pipe that cannot be entered, accompanied by two Goombas and a set of ? Blocks with coins inside.
Lastly, Mario reaches the ending staircase for the level, and the Flagpole afterward, which is reused in all underground and underwater stages in the game.
Mario/Luigi starts the level by falling through a hole in the roof by a pipe. There will be two Goombas and five Question Blocks. The first one contains a Super Mushroom or Fire Flower, the other four ones contain coins. There will be a block tower structure, where there's a Goomba and a Brick Block with 10 Coins. Ahead, there will be some Brick Blocks, one of them containing a Star and some Coins, with two Koopa Troopas underneath them. The player will reach a place filled with Brick Blocks with a Koopa Troopa and five Goombas. There will also be a Brick Block with a Super Mushroom or Fire Flower and a masked Brick Block with ten Coins. There will be a Pit and, after it, a Block platform that allows Mario to get an 1-Up Mushroom. Mario/Luigi will find two Goombas and three pipes, all of them containing Piranha Plants. The player can enter in the first Pipe, that will lead them to a room with Coins and a 10-Coin Brick Block. Mario can exit the area by a Warp Pipe in the extreme right. This pipe will lead to the third one in the main area. There will be two gaps and, between them, two ground blocks and some Brick Blocks. After that, a small, half-pyramid structure with two Goombas is seen. Ahead, a group of platforms will be rising and falling. Between them, a Koopa Troopa and some Brick Blocks. The player will reach a Pipe that will lead them to the final area. They can use the platforms to reach a Warp Zone hidden through the roof. From right to left, the first pipe will head to World 2, the second one will lead to World 3, and the third one will lead to World 4. If the player goes through the regular level pipe, they will lead Mario/Luigi to a pipe with a Piranha Plant in an overworld area. There will be a Block half-pyramid that allows Mario/Luigi to jump on the top of the Flagpole.
Through a small ground area, Mario will reach a giant mushroom that can be used to reach two more mushrooms, containing a Koopa Troopa and three coins. Ahead, three mushrooms are used by two Goombas. On the top of the third and taller mushroom, a shorter mushroom can be reached. There will be a platform that can help Mario to reach the top of a tall mushroom with four Coins. Under this tree, a shorter one contains the only Question Block of the level that contains a Super Mushroom or a Fire Flower, depending on Mario's form. There will be a short mushroom and a taller one, that is used to reach a tall mushroom that contains a Goomba. Between these two mushrooms, there's a Koopa Paratroopa. There will be two platforms that will go side to side. There will also be some Coins. There will be two mushrooms, the first one is small and of medium width, while the other is wide and tall. There will be a Koopa Troopa in the taller one. A short mushroom with three coins is under a Koopa Paratroopa. There will be two similar mushrooms that can be used to reach a platform. The platform can help Mario to reach a Block wall. The Flagpole will be after it.
Mario starts the level on an elevated area with stairs heading downwards. There is a gap with lava and, right after it, a block. There are also two lava gaps with a Firebar on the platform between them. It contains a Question Block with a Fire Flower in it. Past it, a corridor with three Firebars must be crossed. Mario reaches a room with three Firebars, two on the floor and one on the roof, along two blocks. He, then, finds another room with six hidden blocks. In this area, False Bowser's fireballs start to appear. Mario can use a gap to avoid these fireballs and then, the False Bowser is seen on a bridge with a small platform moving from side to side right above him. If Mario gets past the False Bowser by either using the platform or running under him and touches the Ax, the bridge collapses, causing him to fall into the lava. A Mushroom Retainer is saved and Mario gains access to World 2.