The first thing you'll have to do is beat Heat Man's stage so that you'll get Item-1. Now, I know what you're probably thinking: Heat Man's stage is very difficult to beat without using the Item-2 that you get from Air Man. Well, it only took me a few practice tries before I was able to do it, but in case you can't for some reason, here's a password that gives you everyone defeated but Air Man and four energy tanks:
A5, B4, C1, C3, C5, D4, D5, E1, E2
Now enter Air Man's stage and play until you get to him. Make sure you have at least one life in reserve before entering his room. (If you don't, try fighting the birds that constantly respawn just to the left of Air Man's door until you get a 1-Up). Now enter Air Man's room, but DO NOT kill him! Instead, place an Item-1 near the front of the room so that Mega Man rides up alongside the entrance gate like this:
Now it could take a few tries, or it might happen on your first try, but eventually, what should happen is that the Item-1 will trigger the boss gate to open! The action will freeze, the sprites all disappear, and you'll hear the sound of a gate opening, but instead of the actual door opening, a portion of the RIGHT side of the wall opens up like a gate, as seen in this picture:
The game acts as though you just entered the boss room, so it slides one screen over. But since you were already in the boss room, the game has to call up the next location in memory, which happens to be the second level of Dr. Wily's Skull Castle!! But the graphics are still being pulled from Air Man's stage, therefore it's all scrambled and difficult to see:
Now here is why you needed to have at least one extra life. Although Air Man is not in this room with you, his tornado blasts still continue to come out and you'll still get blown around. (Either Air Man is invisible, or his tornadoes are programmed independantly of him and his moves are simply timed in accordance with them.) No matter where you go, you can't outrun the wind blasts, and you'll eventually get pushed onto a bed of spikes and die. But if you have a life in reserve and continue, you'll begin at the start of this room and there will be no more tornadoes. Now you can explore it.
When you reach this point, those "waves" on the floor are actually spikes. Rick L noted in his description that you cannot get past this point without the Item-2 that you get by defeating Air Man. Seems a cruel twist of irony, BUT...I actually have gotten past this point! However, I've only done it with a Game Genie code. Here's the explanation for what I do:
First of all, refill your Item-1 by fighting the enemies in this area (I'll call them "Flyboys" because that's what the ending of Mega Man 2 for the Gameboy calls them). Now wait at the edge of the spike pit and let one of those Flyboys leap into the pit. Leap onto the Flyboy to make yourself temporarily invincible, then place an Item-1 and leap to it. Wait for the Flyboy to leap close, then fall onto him for temporarily invicibility again, then place another Item-1. By repeating these steps over and over again, it's possible to get all the way across the pit to the ladders at the end. There are only a few major problems:
Problem #1: You have to keep hitting the Flyboys to stay invincible, but Mega Man can only take so many hits. It may be possible to refill your energy by using an E-Tank, but this is very difficult to do because you can't access your inventory menu while an Item-1 is onscreen and it's hard to refill while you're on the spikes without getting killed. This is why I have only been able to do this by using a Game Genie code that grants infinite energy:
Note that this code does NOT protect you from the spikes! Only from collision with enemies and enemy bullets. (This, I believe, is because spikes are coded to act the same as bottomless pits when you touch them.)
Problem #2: The Item-1 has a finite energy supply, but it's pretty generous. If you're careful, you can definitely make it all the way to the end (even to the second ladder) and still have a bit of Item-1 energy left over. One thing that helps greatly is to run all the way over to where the low ceiling is before placing the next Item-1 after you jump onto the first Flyboy. Because the area before the low ceiling is wide open, the Flyboy can jump the whole distance. The tricky part is getting through the low ceiling area because the Flyboys can only leap short distances there. If you get too far ahead of them, they won't leap close enough, and you won't be able to jump or fall onto them. When Item-1 gets too close to the ceiling, it'll disappear and Mega Man will fall to his doom on the spikes.
Problem #3: If you don't have Crash Man's weapon (though you will if you used the password I provided above), you MUST take the second ladder because a wall blocks the path after the first ladder.
If all goes well, you'll end up at the ladder here, and from here you can go explore the rest of the stage, but you'll become stuck at the end (explanation on that later). For now, here are some more screenshots:
Be careful, because the "waves" in the third picture are spikes again.
The enemy and power-up item sprites load and act fine. I always did think it was weird how the chains for those metal mashers actually appear at the bottom of the screen. This glitch makes it easier to see them.
Once again, the "waves" are spikes, but if you've played this game enough, you probably know the layout of this level by now.
Guess what? This is as far as you can go. It's the boss room, but the game doesn't load the boss battle and there's no way to move on from here. You can't kill yourself in this room, either, so the only way out now is to reset. Why doesn't the boss battle start? I'm guessing that it's because technically the game still "thinks" you're in Air Man's level. This is evidenced by the fact that the game still plays Air Man's music throughout this whole section (even if you continue) and if you lose all your lives and continue, you start at the beginning of Air Man's stage, whereas I think if the game "knew" you were actually in Dr. Wily Level 2, it would restart you in Dr. Wily Level 2.
And while I used a ROM to get these screen captures, this absolutely CAN be done on the cartridge. I got the glitch to work on my copy of the game, but I didn't get past the bed of spikes because I don't own an NES Game Genie.