Get the national dex and beat the elite 4.
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Complete the quests on the Sevii Islands.
Unknown Dungeon was the name of a cave in Red, Blue and Yellow version that is located northwest of Cerulean City you would need Surf to reach their the cave is now called Cerulean Cave.
get the complete pokedex from prof. oak. go to the sevvi islands and do missions there. get mewto in cerulean cave. complete your pokedex.
You can capture Mewtwo in Cerulean Cave B1, Zapdos in the Power Plant, Articuno in Seafoam Islands B4, and Moltres at Mt. Ember Peak in FireRed.
if can go to cerulean cave you can