The two walked slowly along the woodland trail behind the mountain.
Senku led the way with practiced familiarity, while Li Xiang followed quietly, studying their surroundings. Before long, a pitch-black opening appeared at the edge of the woods ahead.
It was a natural limestone cave.
The entrance lay hidden among thick vines and weeds, so dark that nothing could be seen inside.
Cool air drifted out, carrying the damp, chilly smell of earth, utterly unlike the warm air of the woods outside.
Li Xiang stopped and looked into the deep darkness. "A limestone cave?" she asked softly.
Senku gave a slight nod and kept walking, heading straight inside.
"Yeah. Come take a look. There's something crucial here that we'll need next."
Li Xiang followed him into the cave.
The moment she stepped inside, the light faded, leaving everything dim.
The rocky ceiling was uneven, and water dripped steadily through its cracks. Each drop rang out clearly in the silent cave.
Passages branched in every direction, stretching deep underground. Darkness swallowed their view completely, making it impossible to see the cave's full extent.
By the faint daylight filtering through the entrance, Li Xiang glanced up and immediately noticed the densely packed black shapes hanging overhead.
A huge colony of bats hung upside down from the rocky ceiling, huddled together in silent rest. Not one stirred, and the dark mass felt strangely oppressive.
She suppressed her faint unease and continued forward, quickly scanning the cave.
Soon, an out-of-place object in the middle of the cave caught her attention.
An old cup sat in the center of a clearing strewn with rocks.
It was half-filled with an unidentified cloudy liquid, dark yellow in color, with tiny impurities floating on its surface as it sat undisturbed in the container.
A faint, foul smell of decay drifted over, mixing with the damp scent of earth and stone into something particularly pungent.
Li Xiang sniffed slightly and immediately identified its source.
"It's liquid produced by accumulated animal droppings fermenting."
She turned to Senku beside her.
A familiar, confident smile curled at the corners of his mouth.
"Exactly. This is nitric acid."
He walked slowly over to the cup and looked down at the cloudy solution.
"When I first ended up in these woods, I stumbled across this cave. Animal droppings had been accumulating for years, while minerals in the rock kept dissolving as water seeped through them. Over time, low-concentration nitric acid formed here naturally."
"I also owe my awakening and escape from petrification to the constant soaking and catalytic action of this natural nitric acid."
Li Xiang listened carefully, then calmly pointed out the shortcomings. "Nitric acid formed naturally from droppings is too dilute."
"The nitric acid content is low, and its oxidizing power is nowhere near strong enough to meet the requirements of our later experiments."
She turned toward Senku. "Since you brought me here specifically, you must already have a way to increase its oxidizing power and refine it, right?"
"Yep, exactly."
Senku answered readily, a sly glint flashing in his eyes. "So, I'm going to make wine."
Li Xiang immediately understood his intention and nodded in agreement.
"Excellent idea."
"Nitric acid is a strong oxidizing agent, while alcohol is a flammable organic solvent. Mixing them causes a violent redox reaction that instantly releases a large amount of heat."
"But the risk is enormous. It's a highly dangerous reaction. Combining them can easily produce extremely unstable substances like ethyl nitrate, while rapidly generating large amounts of high-pressure gas."
"If we do it in an enclosed space, the gas can't escape, and the internal pressure will skyrocket, potentially causing an explosion at any moment. Fortunately, the natural nitric acid in this cave is extremely dilute, which greatly reduces the risk. As long as we carefully control the entire process and avoid enclosed conditions, we can handle it safely."
With the principles and risks addressed, the next issue was raw materials.
Li Xiang looked out toward the lush woods and asked a question.
"What about fruit for fermentation? Are there suitable wild fruits here?"
"The simplest way to make wine is to pick fresh wild fruit, mash it, and seal it up to ferment naturally. That'll give us a low-alcohol fruit wine, enough for our experiments."
"We just need to look," Senku replied casually.
They turned and left the chilly cave, returning to the sunlit woods. The vegetation behind the mountain was especially lush, with fertile soil, plentiful water, and abundant wild plants and animals.
They searched carefully along the hillside. Before they had gone far, they found a large patch of wild grapevines on a gentle, sunny slope.
The vines sprawled across the entire slope, their leaves lush and green. Clusters of plump, round, purple-black wild grapes hung among them, weighing down the branches in abundance.
"These will do."
They immediately began picking, working briskly and dividing the tasks between them. Cluster after cluster came off the vines, and before long, they had filled a large basket, more than enough for making wine.
With their haul, they returned to the clearing beside the wooden cabin.
Li Xiang found a clean stone vessel and poured all the wild grapes into it.
She crushed the plump grapes thoroughly with her fingers, mixing the flesh, juice, and skins together. A sweet, tart fruit scent immediately spread through the air, dispelling the lingering stench from the cave.
Senku helped prepare the containers for sealing and clear away impurities, the two working in easy harmony throughout.
Once all the grapes had been thoroughly crushed and mixed, they worked together to pour the pulp and juice into earthenware jars, sealing them tightly to keep out the outside air, then leaving them to ferment naturally.
With everything finished, Li Xiang wiped the fine beads of sweat from her forehead and let out a long breath.
"Wild fruit wine doesn't take too long to ferment. Leave it at room temperature for about two weeks, and we'll have usable low-alcohol wine."
Over the following days, they stopped rushing to advance their acid-making experiments and used the time while the wine fermented to make steady progress on reinforcing the house.
Senku planned the work, selected limestone, prepared the volcanic ash, and oversaw all the scientific proportions and procedural details.
Li Xiang handled most of the physical labor. There was no counting on Senku for that.
Who would've thought I'd be doing manual labor here as a medical student?
With their orderly cooperation, the work they had originally estimated would take a month progressed steadily, putting them a little further ahead each day.
Their days were simple but full, and time in the mountain wilderness passed quietly and slowly.
Several days slipped by, and evening approached.
The sunset faded, and night slowly settled over the woods. The day's heat receded, leaving the evening breeze cool and gentle.
The black night sky spread overhead, and countless stars lit up one after another, filling it with clear, brilliant light that spilled across the desolate earth.
The woods fell quiet. Insects chirped softly, and the wind whispered, the bustle of the day's work giving way to boundless stillness.
Li Xiang stopped working, straightened, and lifted her eyes to the dazzling stars overhead.
The starlight reflected in her clear, obsidian-dark eyes. Quiet and gentle, they had lost the sharp efficiency she usually wore while working, softened by a faint tenderness.
After a long silence, she spoke softly, her voice carried far by the evening breeze.
"Little Senku, do you miss home?"
Senku paused at her words, his hands going still over the stones he had been sorting.
He looked up at the night sky, and familiar faces flashed through his mind.
His kind, easygoing dad, the cheerful Taiju, the lively, dependable Yuzuriha, and that modern world of flourishing civilization and bustling everyday life.
Thousands of years of petrification had cut him off from everyone he loved and everything he had known.
Through that long, lonely dormancy, no one had known, and no one had been there with him.
Senku remained silent for a moment, then nodded gently.
"Yeah. Of course I miss home."
Rebuilding civilization and saving the world had never been just some hot-blooded slogan.
It was the only way he could go home and see the people he knew again.
After hearing his answer, Li Xiang said nothing more.
The evening breeze gently brushed the hems of their clothes. The stars shone brilliantly, and the mountain wilderness was silent.
Before you continue