The Pokémon Mansion of Cinnabar Island was the home of a team of researchers, led by a Pokémon collector simply named Dr. Fuji. It was also the birthplace of the Legendary Pokémon Mewtwo. This is the story (rather, my own interpretation) of how a place of modern science and new ideas became an abandoned husk riddled with regret and broken dreams.
Dr. Fuji, wanting to find a way to recreate life, embarked on an expedition to Faraway Island, a place yet to be found on a map. He and his colleagues journeyed through the island's dense jungles, searching for a sign of Mew, an extremely powerful Mythical Pokémon long believed to be extinct. Giovanni, leader of the crime syndicate Team Rocket, had learned of Fuji's work in the field of genetics and agreed to fund the journey and their research, but only if they would artificially engineer a powerful new Pokémon. Ultimately, Fuji chose to agree. For as far as he knew, Team Rocket simply wanted to control a super weapon for their criminal empire. Fuji wanted something much more precious: the key to restoring life itself.
After nearly a week in the Faraway Island wilderness, Dr. Fuji's team discovered what they believed to be a fossilized remnant of Mew. With any luck, if it was real, the item would have the sufficient genetic material for them to create a Pokémon with enough life energy to survive the cloning process. Seizing the moment, the crew returned home immediately to begin preparations.
Months passed after the discovery. The laboratory underneath the Pokémon Mansion was at its busiest. Through tireless nights of gene-splicing experiments, at least five specimens were gestated and contained in suspended animation. In the end, only one survived. By early February, Dr. Fuji's findings had proven successful. From the DNA within the Mew fossil, the scientists managed to create an entirely new Pokémon: Mewtwo.
For some time, things seemed to be looking up for everyone involved. Mewtwo was in stable condition and growing stronger every day. Soon, Giovanni would have the most powerful Pokémon in the world. And Dr. Fuji and his team seemed to finally have the answer to return long-gone beings to life. A win-win scenario. However, amidst the months of genetic tinkering, the scientists had accidentally created a deadly super weapon of their own: a dangerous chemical known as the Berserk Gene. It was discovered that when a Pokémon ingested the mixture, its power would increase to extremely high levels. Unfortunately, this also put the Pokémon in a crazed state, becoming highly aggressive and later left weakened and confused, with no memory of what happened. Upon learning this, Fuji had the Berserk Gene stored away with the hopes that it would never be used again.
One day, however, the concoction was accidentally dropped into Mewtwo's container during an examination. Its vital signs skyrocketed and its brainwaves went out of control. If the gestation process was interrupted now, Mewtwo would die. Acting quickly, Fuji ordered for it to be sedated before it could be awakened prematurely. The sedation worked almost immediately. Its brainwaves went back to normal, and everything seemed alright. Fuji was relieved, but was secretly worried. The Berserk Gene was gone forever, but one drop was enough to drive a Pokémon into a destructive rampage. Mewtwo had absorbed all of it.
Months of dedication and further experimenting passed. In time, Mewtwo slowly gained consciousness, monitored around the clock. It paid little heed to any Pokémon that were brought into the laboratory, but exhibited very unusual behavior towards the scientists, staring coldly whenever they passed it by. Its savage temperament escalated into acts of resistance and aggression, banging on its glass container from the inside. Dr. Fuji knew that Mewtwo was growing not only in power, but in self-awareness. It didn't belong there. It was brought into a world that seemed to only offer misery, for the sake of scientific curiosity. Come the beginning of September, the Psychic time bomb would finally explode.
The devastation was swift and immense. The laboratory was destroyed by fire, and everyone who lived in the mansion was gone, never to return. Mewtwo had escaped.
The Pokémon Mansion was never the same again. Wild Pokémon now wander the abandoned halls, and burglars have been caught attempting to steal the mansion's spoils. Team Rocket cut all ties with the establishment after learning of Mewtwo's escape, the world's most powerful Pokémon having been lost. Blaine, Gym Leader of Cinnabar Island and longtime friend of Dr. Fuji, was the only known survivor of the incident. He was an on-and-off assistant for the creation of Mewtwo, even lending a little bit of his DNA for the cloning process, but was not present to witness what came next. Dr. Fuji's research was dismissed after everything that transpired. No one has seen or heard from him since, but some are sure that somewhere, he's still alive. Mewtwo lives in solitude as a wild Pokémon, only thinking of defeating its foes. It secluded itself in Cerulean Cave, before finally leaving the Kanto region for good. It was most recently sighted in the Kalos region, having settled somewhere near the hidden Pokémon Village. It is believed that there is an unknown dungeon where it lives to this day, awaiting the day an equally strong opponent will challenge it to battle.