(This code is a rumor, but two of my friends said they have been to it)
First, Beat the Diglet/dugtrio and Gengar (in red) or the Meowth and Seel (in blue) Bonus Stages. Now, go to the Mewtwo Stage. Defeat Mewtwo. KEEP YOUR GAME ON! Now, on the field, get more than 1,000,000 points. If you lose all your lives, it is OK, just go back to the field where you were in. You will have the same amount of points when you ran out of lives. When you go beyond 1,000,000, thescreen will freeze for a second, flash, and then the rumble feature will start going. Music will play and a special screen will show up on the info/bonus/CAVE screen. The hole will open up, and when you go into it, you will go into a special Chronamew stage. You will have to dodge the psychic waves that Chronamew fires at you. Hit him 10 times and you will have caught Chronamew!