From Bulbapedia, the community-driven Pokémon encyclopedia.
The Pokémon Mansion journals are a series of diary entries written by a Pokémon researcher (possibly Dr. Fuji) chronicling the events that led up to the creation and escape of Mewtwo. They are scattered around the Pokémon Mansion on Cinnabar Island, the only remnants of the research that went into Mewtwo's creation.
Guyana, South America. A new Pokémon was discovered deep in the jungle.
We christened the newly discovered Pokémon Mew.
Mew gave birth. We named the newborn Mewtwo.
Mewtwo is far too powerful. We have failed to curb its vicious tendencies...