Unknown Dungeon

It is the cave located north west of Cerulean city with a person standing at the entrance. You can only go in after you beat the Elite four and Gary. The Pokémon in the cave are very high levelled and Mewtwo is somewhere deep inside…

Snorlax, Legendary Birds and Mewtwo

These Pokémon all have one thing in common: you only get one chance to capture them (two for Snorlax). You will fight Snorlax by playing the Pokéflute, and the others are just standing there until you talk to them. Always save before doing this! If you accidentally kill it or you run away from the battle, you will never have another chance to capture it.