This cannon will take you to the Flower world. Clear that level and you can now access the cannon. Above that path there will be a P switch. Push the middle and right one and proceed through the top pathway. Go to the Ghost House where you pick up the third star coin. The Flower World can also be reached from the cannon in the Mushroom World.

Make it to the flagpole to unlock the Warp Cannon that will take you to the Flower World. Survive through the rest of the level, and don't let yourself get squished by the tricky blocks you'll find along the way. Follow the right block to find a Mini Warp Pipe. Once you're in the castle, proceed as normal until you come upon the first set of small, square blocks which float up and then separate (this occurs fairly early on in the level). To do this, go to the first castle in that world - but you'll need to be Mini Mario for it to work, You can grab a couple of Mini Mushrooms in World 3-B. To get to the Flower World, you need to unlock the Warp Cannon in World 3. Dash your way through the Warp Cannon level to unlock the Mushroom World.

Hit the flagpole, and when you're taken back to the Map the road to the Warp Cannon will unlock.

Hurry, and be careful not to get yourself squished! You'll be taken to a room with a Golden Pipe - go through the pipe to get to the very end of the level.

Go through the door and follow the Snake Blocks through the hole on the right of the screen. Just after the Midway Flag, you'll see a door. To do this, go to the first castle(Or tower) in that world. To get to the Mushroom World, you need to unlock the Warp Cannon in World 1.

Super mario bros 1 world 1-2 secrets