A guide to completing World 1-2 in Super Mario Bros 1 for the Nintendo Entertainment System.
Explore World 1-2
After a brief cutscene, you will enter the underground area of World 1-2.
The leftmost ?-mark block contains a Fire Flower or Mushroom, so get that, then hit the remaining ?-mark blocks if you feel like it. Defeat the Goombas on the right and jump over the pillars of blocks. There will be a single floating brick along the way. Hit it to get some coins.
To the right, there is a W-shaped structure. Beneath it, there are two Koopa Troopas, which you can hit with fireballs to get rid of, or jump on top of them. If you do jump on them, their shell will stay there, and if you touch the shell, you will kick it in the direction you're facing, which will defeat any enemies in its path. If you leave its shell sitting there, the Koopa Troopa will eventually come back out of the shell and start walking around again.
Hit the leftmost bricks of the W-shaped structure and you can then jump up to the coin that is sitting there. Note that if you stand on the top of this W-shaped structure, you can jump and hit the ceiling bricks. If you're Super Mario or Fire Mario, you can break the ceiling bricks, then make a running jump to jump on top of those bricks. This lets you skip a large part of this World, which is helpful if you want to try to enter Minus World, but you miss out on coins, points, and a 1UP. If you don't want to use that shortcut, go to the right to get the remaining coins, but before you move on, go down to ground level and hit the rightmost bricks of the structure. A Starman will pop out that you can use to defeat the enemies as you go to the right.
The ceiling to the right is too low if you're Super Mario or Fire Mario, but you can get past it if you hold B to run toward it, then crouch just before you hit the wall. You'll slide partway under the wall, and if you slid far enough to the right, you can let go of the crouch button and you will slide out on the right side. If you didn't slide far enough under the bricks, you can press B to jump while crouching and break the bricks.
After you get past the low wall, jump up to get the coins, then go back down and continue going to the right. If you're Fire Mario and don't mind missing out on some points, or if you're regular-sized Mario, just hit the brick beneath the coin to get it. If you're Super Mario, don't hit the brick beneath the coin. Instead, hit the bricks to the left of it until you can jump up and get the coin. Then go back down below, and hit the bricks to the right of where the coin was. The upper brick has a Fire Flower in it.
To the right of that, there is a big thing of floating bricks. If you're Super Mario, you can break the bottom-right brick, then hit the one above that to get coins.
To the right, get on top of the bricks, but be careful of the Goombas up there (if they're still there). Then you can try to run to the right and jump onto the floating bricks to the right of the gap in the floor. Get the coins there, then stand on the rightmost edge of this platform but be careful not to fall off. If you're positioned correctly, you can jump straight up and break the brick to the right of where the rightmost coin was. Then move slightly left and jump to hit the ceiling brick to the left of the hole that you made. a 1UP mushroom will pop out and you can either jump to get it or you can let it fall through the hole you made, and you can chase after it where there are a few Goombas. (If you try to get the 1UP without breaking a hole in the ceiling first, it will just move along the ceiling and you will have to either quickly break a hole in the ceiling and try to make a running jump to go up through it, or you have to be Fire Mario to run and shoot fireballs at all the enemies to the right so you can get the 1UP mushroom when it falls down from the ceiling later, which will cause you to miss some bonus coins.)
Assuming that you are still where you got the 1UP mushroom, go to the right and you find a few Goombas, then a green pipe with a Piranha Plant coming out of it. If you're Fire Mario, you can press B to defeat the Piranha Plant with a fireball. Otherwise, you need to wait until the Piranha Plant goes back down into the pipe. Stand next to the pipe and the plant won't come back up until you move farther away, so stay near the pipe to keep the plant inside. Jump onto the first pipe, and if you want some bonus coins, press down to go down the pipe. Get the coins on the top row, then go down and get the bottom coins, but before you enter the pipe, jump and hit the rightmost brick to get coins. Then press right to enter the pipe. You'll be on the rightmost of the three pipes. (If you skipped the bonus coins, you can just go to the right to reach the rightmost pipe.)
To the right, hold B to make a running jump onto the bricks between the two gaps in the floor. Then go to the right, defeat the two Goombas on the stairs, and you will find platforms moving downward. Jump on a platform when it's still in the upper half of the screen, but be careful when jumping off to the right, because if you hit the platform above, you'll hit your head and might not jump as far as you expect. When you're on solid ground to the right of the moving platforms, go to the right and get beneath the rightmost brick, and jump up to hit it and get a Flower or Mushroom.
Go to the right and get on a platform that is moving up. Before you continue, read this paragraph to decide what you want to do next. To the right, there is a pipe, but there is also a secret area if you skip the pipe. If you're Super Mario or Fire Mario, you can also try to enter "Minus World", a famous glitched level. It can only be entered using a glitch. If you're not interested in Minus World, skip to Choose Your Own Adventure below. Otherwise, keep in mind that could take a while to correctly perform the glitch, so you should only try this if you ran along the ceiling to skip most of this level, giving you plenty of time to attempt the glitch. Keep in mind also that there is no way out of Minus World, and you'll get a Game Over because you'll run out of time from being stuck there. Minus World is an underwater world, and you can swim by tapping the jump button repeatedly.
How to Enter Minus World
If you do want to see Minus World for yourself, remember that you have to be Super Mario or Fire Mario to get there. To do so, don't get on the ceiling. Instead, stand near the entrance of the green pipe, then jump on top of the entrance of the pipe.
Crouch Method
One way to get there is to crouch, then press B to jump and immediately press right so you jump to the right while crouching (note that if you press a direction key while crouching, you'll stand up, so make sure not to press a direction button until after you press B to jump). If you hit the green pipe in the correct location, you will automatically start walking through the tiles on the right until you've gone all the way through. Don't go any farther to the right than necessary to enter the first green pipe. If you do, the words "Welcome to Warp Zone!" will appear on the screen, at which point the pipes will just go to where they're supposed to, instead of taking you to Minus World.
Brick Method
A more difficult method is to stand on the leftmost side of the pipe and jump straight up to break the second and third ceiling bricks from the right. Now, stand on top of the pipe entrance again if you aren't already, and face left. Press B, then press right so you're jumping to the right while facing left. You should see Mario go partly through the ceiling brick on the right (be careful not to actually break that brick; if you do, you'll have to try the crouch method instead). If you fall off the pipe, don't move. Be careful not to accidentally enter the pipe. Your goal is to jump far enough to the right that you go right through the wall and end up inside of the tiles of the pipe. This will cause you to automatically start walking through the tiles on the right until you've gone all the way through. Don't go any farther to the right than necessary to enter the first green pipe. If you do, the words "Welcome to Warp Zone!" will appear on the screen, at which point the pipes will just go to where they're supposed to, instead of taking you to Minus World.
Choose Your Own Adventure
If you decided not to go to Minus World, you can either go into the pipe to the right of the rising platforms to go outside, where you can go up the stairs and jump onto the flagpole to proceed to World 1-3, or you can jump on one of the rising platforms (if you haven't gone too far to the right to do so), then jump onto the ceiling on the right. Keep going to the right and you'll enter the Warp Zone, where you can choose to warp to World 2-1, World 3-1, or World 4-1.