To access this warp, you must be Super or Fiery Mario. Go to the pipe at the end of the level and jump on top of it. Jump up and break the block over top of the part of the pipe farthest left. Then, position Mario on the edge of the pipe, with one foot hanging off the edge. Duck, then jump backwards as far right as possible. If done just right, Mario will slide through the pipe and the bricks. Walk over to the warp zone area and enter the middle pipe. This pipe will take you to world 5, level 1. The same area can be acessed from a pipe in the same place in 4-2. However, this is meant to be a warp zone to 5-1, whereas the 1-2 warp to 5-1 is a glitch. After completing the jump trick in level 1-2, you can also go down either of the other two pipes, both of which will take you to Minus World -1.
If you just walk along the sea floor, Bloopers will never be able to kill you because they are forced back up just before they hit the floor(or you).
After a game is over, go to One Player Mode, then hold A and press Start . The game will resume from the last world played.
To warp to Worlds 2, 3,or 4 during World 1-2, go to the end where the 2 platforms are rising, get on the first platform, and use it to jump onto the highest structure in the middle, then jump on the next rising platform, but this time, when it reaches the top, jump onto the TOP of the level. Walk foward and drop down the small jump, and you can now warp to Worlds 2, 3, or 4 by going down the desired tube, and pressing down.
Get to world 3-1, and become Super Mario. At the end of the level(the staircase before the end flag) you will see two turtles coming down the stairs, jump over the first one and then jump on the second one. It should hit the star and bounce back, from here jump on it and keep yourself up, after a certain number of bounces without hitting the ground, each consecutive bounce will get you a 1-up. Get as many as you want but be warned, if you get over 100 you will get a Game Over.
Warp to World 5
Well this secret is so easy. At the very end place of world 4-2 (where you can see the pipe going to the flag) jump on the long pipe and break the brick above it and go through the end and you can see the pipe going to the World 5.