A hundred meters beneath the Hokage Rock.
Hiruzen Sarutobi slipped through the earth with Earth Style, descending layer by layer. After undoing multiple seals, he stepped into a narrow underground chamber.
The air carried a faint scent of decay. The walls and ceiling were hardened into dark stone by Earth Style, smooth and cold.
Rust-flecked experiment tables stood in rows. Cultivation tanks lined the walls, each filled with different kinds of "materials."
At the far end, one tank drew the eye immediately. Dense layers of sealing jutsu covered its surface. A thick transparent tube extended from its base and connected to another chamber.
This was Tobirama Senju's hidden archive.
Hiruzen had come here before, long ago.
But Tobirama's research ideas had been… unsettling, even to him.
After only a brief review back then, Hiruzen had realized he could neither control nor continue those experiments. He reinforced the seals, took the notes, and locked the equipment away deep underground.
He had always been curious.
It was just that when he first arrived in this world, other matters demanded priority.
Now that he finally had room to move, the Hokage came here without hesitation.
He couldn't help but smile, recalling how, twenty years ago, he nearly blew himself up the first time he triggered Tobirama's defensive arrays.
With the Second Hokage's research notebook in hand, Hiruzen moved through the chamber, checking each astonishing half-finished creation.
"This pool of black sludge… it must be that creature designed to absorb the negative emotions of the human heart. A pity it has no vitality left."
He shook his head in admiration.
Tobirama had once proposed harnessing the negative energy that inevitably accumulated in people's hearts, shaping it into a creature that fed on it, an artificial Tailed Beast.
He had named it the Zero-Tails.
"And there are still some Sharingan here. A single tomoe, a pair of Three Tomoe…"
Likely trophies seized during the Warring States era.
The number was smaller than Hiruzen expected.
After all, Tobirama's notes dissected the Sharingan in painstaking detail.
Its origins. Its structure. Its operational mechanisms. Even methods for awakening it were recorded.
If he were to bring a Three Tomoe from the Uchiha compound now and debate the Sharingan with Tobirama…
Hiruzen was confident his teacher would leave the Uchiha speechless.
At this point, Tobirama might as well have possessed a Mangekyo Sharingan in a previous life. How else could he understand it so thoroughly?
No one understood the Uchiha better than Tobirama Senju.
Hiruzen stepped before the massive tank shaped like a black coffin and looked up at it.
He could guess part of the story.
From the research notes, his own memories, and a bit of deduction, he pieced together the outline.
Before the First Shinobi War even broke out, Tobirama had been considering how to use bioengineering to strengthen Konoha.
His first thought concerned injured shinobi.
Shinobi relied on hand seals. Restoring damaged limbs would have been a righteous goal, aligned with the Will of Fire.
But as the research progressed, Tobirama began contemplating cloning an entire human body.
That part, Hiruzen could understand.
Even a skilled Jonin's body, if repurposed as a puppet or otherwise utilized, would possess formidable combat value.
But Tobirama had failed.
According to his findings, developing a Chakra-bearing shinobi from cells was not merely a matter of genetic replication.
The more talented the original shinobi, the more likely the cloned cells would devolve into chaotic, useless tissue.
Tobirama speculated that this phenomenon was tied to factors such as the "maternal growth environment" and "soul nourishment." He listed numerous variables that might interfere.
With Hiruzen's current knowledge, he agreed.
Unless the original body possessed some unique trait, large-scale replication of shinobi was nearly impossible.
One would have to proceed step by step, making minute adjustments, repeating experiments countless times in hopes of striking the correct configuration by chance.
Perhaps future advances would uncover a solution.
Hiruzen released the outer seals of the black coffin.
Inside lay the body of a youth, around fourteen or fifteen.
Familiar. Yet unfamiliar.
Messy black hair framed a handsome face. The eyes and brows bore some resemblance to Tobirama Senju, but not entirely.
"…Sensei."
Alone in the underground lab, the normally composed Hokage felt the corner of his mouth twitch.
"I never saw him in person, but the target of your Cell Fusion experiment… it must have been the one you kept mentioning in your notes."
Izuna Uchiha.
Hiruzen turned his gaze to the connected chamber.
Within it floated masses of wood-like cells. A brief sensing confirmed it.
Hashirama Cells.
"So you filtered water infused with Hashirama Cells through sealing techniques and circulated it to nourish this body…"
Scientists always kept records.
And Tobirama Senju, who died covering the rear on the battlefield without clearing his "browsing history," had left everything for Hiruzen to read.
He had read it thoroughly. And with great interest.
Staring at the experimental subject, Hiruzen's mind leapt.
Fragments from the notebook linked together into a story.
One day, Tobirama pierced his enemy's chest with Flying Raijin Slice and tore away a piece of flesh.
He preserved it.
He cultivated it.
After becoming Hokage and gaining access to resources, perhaps driven by some complex impulse, he experimented with cultivating his enemy's cells.
Then he discovered something astonishing. When he incorporated his own cells, experiments that had always failed began to show signs of success.
After untold adjustments, refinements, and repetitions, he had actually produced a complete human body.
He stabilized it within the tank using multiple layers of sealing jutsu.
Waiting to return from the battlefield and continue the research.
Only…
Hiruzen's gaze lingered on the joints of the body.
There were flaws.
Subtle nodules, like those found in someone whose meridians had once been severed.
Having practically memorized the notes, he immediately recalled the unresolved obstacle.
"How can the auxiliary meridians be made to conduct smoothly?"
"Mito might know… but asking her would reveal my intentions."
"Troublesome. My brother has repeatedly told her to keep an eye on my research."
The notebook contained no such sentences. Only the phrases "meridian conduction" and Mito's name scratched out roughly.
Yet in Hiruzen's mind, the scene was vivid.
Whether true or not, he would reconstruct it this way.
The dead could not refute him.
Hiruzen felt his teeth ache.
What Tobirama had done to Izuna Uchiha went beyond hatred. Even "fight to the death" felt inadequate.
If one altered the genders, it could dominate the charts as a tragic romance.
The entanglement between the Senju and the Uchiha was impossible to untangle.
"I was still wondering about a suitable body. Now there's one ready-made. It only needs refinement."
"Sensei, the questions you didn't dare ask Lady Mito… I'll ask them for you."
The Hokage of Konoha resealed the tank.
Without him, this research might have remained buried forever beneath Konoha.
As years passed, it might collapse. Or some future Hokage might erase it entirely as forbidden history.
But now, things were different.
Konoha had to accelerate.