A hundred meters beneath the Hokage Rock.
Hiruzen Sarutobi slipped down through Earth Style, peeled through layer after layer of seals, and entered a long, narrow underground chamber.
The air carried a faint, rotten tang. The walls and ceiling were hardened into dark stone, packed tight by Earth Style until it looked almost ink-black.
Rust-stained lab tables sat in lines. Along the walls, cultivation tanks stood like sentries, each one holding some kind of "material."
The farthest tank drew the eye immediately. Dense sealing scripts crawled over it in overlapping sheets, and a thick, transparent tube extended from its base into a second storage unit.
This was Tobirama Senju's hidden vault.
Hiruzen had been here once, long ago.
Back then, Tobirama's research had been… too much. The ideas were so extreme they made the skin itch.
Hiruzen had flipped through a few things, realized he couldn't control it and couldn't continue it, reinforced the seals, took the notebooks, and buried the rest of the equipment down here under the village.
He'd been interested ever since.
But when he'd first arrived in this shinobi world, it hadn't been something he could put first.
Now that he finally had room to move, the Hokage came down here with real excitement.
He remembered his first visit twenty years ago, when he'd nearly been blown apart because he didn't understand Tobirama's defensive formations yet, and he couldn't help a small smile.
With Tobirama's research notes in hand, Hiruzen walked the room and checked each half-finished horror like he was taking inventory.
"This black sludge…" he murmured, crouching near a sealed container. "That should be the monster meant to feed on negative human emotion. Too bad it's dead."
Tobirama had once proposed a concept, using the dark energy that always rose from people's hearts to create a creature that could survive on it, a kind of artificial tailed beast.
He'd called it the Zero-Tails.
"And there are Sharingan here too," Hiruzen said, moving along the row. "Single tomoe… a pair of Three Tomoe…"
Spoils from the Warring States Era, most likely.
Still, the quantity was smaller than he expected.
Tobirama's notes on the Sharingan were almost absurdly detailed. Cause, structure, specific mechanisms, even methods for awakening it, all laid out in clean, sharp lines.
If Hiruzen pulled a Three Tomoe from the Uchiha compound right now and let Tobirama argue the Sharingan with him…
Hiruzen was confident his teacher could talk the man into complete surrender.
At minimum, Tobirama had to have been a Mangekyo Sharingan in another life, because how else did someone understand it this well?
One thing was certain.
No one understood the Uchiha better than Tobirama Senju.
Hiruzen stopped in front of the tank that looked like a black coffin and lifted his gaze.
He could already guess part of it.
Between the research notes, his own old memory, and a little bit of inference, the outline came together.
Before the First Shinobi World War, Tobirama had already been thinking about how to use biological engineering to strengthen Konoha.
At first, it had been about injured shinobi.
Shinobi needed their hands for hand seals. Restoring limbs to crippled shinobi, giving them their bodies back, that was clean in purpose and intent, the kind of "right path" that fit the Will of Fire.
But the longer the work continued, the more Tobirama's thinking shifted.
He started wondering about cloning an entire person.
Hiruzen could understand that too.
Forget Hashirama Senju and Tobirama Senju. Even an elite Jonin's body, even as something like a puppet base, would be real combat power.
But Tobirama had clearly failed.
His results showed that cloning a shinobi with Chakra wasn't as simple as copying a genome and letting it grow.
The more talented the shinobi the cells came from, the more likely the clone attempt would devolve into useless, chaotic tissue.
Tobirama had suspected the cause might be tied to "the mother body's growth environment" and "nourishment of the soul." He'd listed a disturbing number of interfering factors.
With what Hiruzen knew now, he and Tobirama shared the same conclusion.
Unless the target body had some special trait, large-scale shinobi cloning wasn't something you could just replicate on command.
You could only adjust, test, adjust again, and keep testing, hoping that enough attempts would eventually brush the right answer by chance.
Maybe one day, when the technology advanced further, there would be a solution.
Hiruzen unsealed the outer bindings of the black-coffin tank.
And what appeared before him was a boy's body, familiar and unfamiliar at the same time, fourteen or fifteen at most.
Messy black hair. Handsome features. The brows and eyes carried a faint resemblance to Tobirama, but not fully.
"Hah…" Alone in the lab, even Hiruzen couldn't keep his mouth from twitching. "Sensei, I've never seen him in person, but I'm guessing the fusion target you kept mentioning in your notes was that man."
Uchiha Izuna.
Hiruzen tilted his head toward the unit connected by the thick tube.
Inside were masses of cells that looked almost like wood. A single touch of Chakra was enough to confirm it.
Hashirama Cells.
"Filtering water infused with Hashirama Cells through seals, then cycling it to nourish this body," Hiruzen murmured. "So that was your solution."
Whether ordinary people kept diaries was debatable.
Researchers, though, always left records.
Tobirama had died covering the retreat on the battlefield, with no time to erase his "life browsing history," and Hiruzen, inheriting everything, got to read it all.
And he'd read it with real interest.
Standing over the tank, Hiruzen's mind started building connections, stitching fragments from the notes into a story.
One day, Tobirama pierced an enemy's chest with Flying Raijin Slice and tore free a piece of flesh.
He preserved it. Began cultivating it.
Later, once he became Hokage and had resources, he ran an experiment. For some strange motive, he tried cultivating cells from an old enemy.
And when he added his own cells, the experiments that had always failed finally showed a turning point.
After countless micro-adjustments, countless shifts in ratios and conditions, he actually produced a formed human body.
He locked it in a cultivation tank under multiple layers of seals.
He meant to return from the battlefield and continue the research…
Only…
Hiruzen's gaze dropped to the joints. There were defects there, little knots, like the swelling you saw in someone whose Chakra pathways were severed.
He'd practically memorized Tobirama's notes by now. He immediately recalled the bottleneck.
What could make the Chakra pathways conduct smoothly?
Maybe Mito Uzumaki knows, but if I ask her, she'll notice what I'm thinking.
Annoying. Big Brother told her more than once to keep an eye on my research.
Those exact lines weren't in the notebook. What was there were only rough scribbles, "Chakra pathway conduction," and Mito's name scratched out in a hurried mess.
But Hiruzen's eyes felt like they were looking through time, catching the shape of Tobirama's thoughts back then.
Whether it was true or not, Hiruzen decided, he'd run with it.
The dead didn't get to argue.
Hiruzen clicked his tongue, almost aching.
What Tobirama had done to Izuna…
"Blood feud" didn't even begin to cover it.
Swap the genders and it would read like the cruelest tragic romance you could submit to any magazine and take the top spot.
The Senju and Uchiha really were something else.
"I was still thinking about bodies," Hiruzen muttered. "Now there's one right in front of me. Convenient. It just needs some work."
"And if you didn't dare ask Mito-sama…" He smiled faintly, cold and amused all at once. "Then I'll ask for you."
The Hokage slowly sealed the tank again.
If he hadn't come, this research might've stayed buried under Konoha forever.
Time would pass. The chamber might collapse. Or some future Hokage might bury it as a forbidden historical zone until it vanished.
But now it was different.
Konoha had to start accelerating.