Electrician

To get the real ending of the game, you need to follow certain conditions. First, you have to change the backgrounds of the sewer levels. This is achieved by simply jumping in specific spots and you'll immediately see an animation or a background change. Second, you have to finish the game 8 times. Each time you beat the game, if you successfully managed to change the backgrounds in the previous sewer levels, you'll get a replacement sewer level (one per loop). You'll know you did it right if the game prompts you to change disk sides. On the first run you'll get the first alternative sewer level in world 5. On the second run you'll get it in world 6. On the third run you'll get it in world 7. On the fourth run you get no alternative levels. On the fifth run you'll get it in world 1. On the sixth run you'll get it in world 2. On the seventh run you'll get it in world 3. And on the eighth run you'll get it in world 4, which will also be the last stage if you did everything right. In short, just change the backgrounds on ALL the sewer levels until you beat the game 8 times, if done right, you'll get the real ending.