This is the Pipe Maze world. It is a big maze that you're going to have to use several strategies for.
7-1
This level is simply a level where you can go to the left and you'll end up on the right side of the screen once again. Get all the way to the top and enter the pipe and you'll get to the goal panel.
7-2
This level is a desert pipe maze. Most of the pipes are solid, but some will take you to rooms. You must get a set of Jump Blocks that you can cross over. At the end, of course, is the goal panel.
7-3
You can simply starman your way through this level. There are a total of 6 starman blocks, but except the first one, you must already have starman to get it, otherwise, all you'll get is coins!
7-4
If you have a P-wing, you can fly over the wall to get a pair of 1-Ups. You can not, however, get to the goal panel simply by flying over the next wall. Hammer Mario is a good friend of yours in this side-scrolling water realm. Hammers can kill just about all the enemies, even Jelectros and Lava Lotuses, but otherwise you'll have to simply avoid them. In the middle of the level, the screen will scroll vertically.
7-5
This is a maze with several rooms. Follow the pipes and try to search for a route that no invisible blocks will block you from getting to. Located in the middle of the level is a group of Ice Blocks with a Troopa inside them, just after a pipe.
(Variant: In Super Mario All-Stars, the ice blocks were moved one step to the right to prevent a glitch; see the info in 3-9 for what it is.)
Piranha Plant
Just follow to your right and avoid the plants. You'll be in a secret room and get a P-wing!
Fortress
Raccoon Mario is necessary for this fortress. You can get into the second room either by entering the door at the end, or by finding a P-switch to turn all the bricks into coins and going to the bottom and entering that door. Either way, you'll end up in the same room. In this room, go to the right and don't enter the door at the very end. Instead, when you get there, fly to the left and enter the vertical pipe to get to Boom Boom's room.
7-6
If you have a P-wing, you can simply fly over the wall and get to the end of the level. Otherwise, enter the door and you'll be in a room similar to World 4's second fortress. Get up to the pipe, because of the spiked teeth, you'll need to be on a lift to get to it.
7-7
This level is just a line of Munchers. You need a starman to get by them.
7-8
This level has lots of enemies. It also has a pink block that will take you to a sky room identical to the one in 3-7 except that it ends with a pipe that leads you back to the main level. It also has a secret room with a Hammer Suit in it.
7-9
In this desert level, you just search as hard as you can for the root to the exit pipe.
Fortress
If you're Hammer Mario, you can practice killing enemies just like in World 6's third fortress. There are no secret rooms in this fortress, just enter the Pipe at the end and get to Boom Boom's room.
Piranha Plant
Just like the first one. Unfortunately, unlike the previous piranha plant level, the prize you get is a not-so-exciting Super Mushroom.
Airship
The King has been transformed into a Piranha Plant. Use bolt lifts here. At the end is a strong boss who is very hard if you don't have a P-wing. At the end, after getting the wand, the king will lie. He says "Here is a letter from the Princess", but it is not, it is from King Koopa, saying he kidnapped the princess!