On December 19th, Jin and Elsa hurried through breakfast and headed straight to the Tatsumi Bay docks. The time had finally come to board their submarine and begin their deep-sea exploration.
"Jin-nii, what is this ship’s name?" Elsa asked, realizing the vessel was still anonymous just as they were about to depart.
"Hmm, you're right. It needs a formal name. Calling it 'Submarine' is a bit too literal, and 'U-boat' wouldn't be appropriate..." Jin mused, falling into his usual habit of overthinking. "How about... the Shachi?"
"...I like it."
"Alright. Let’s give it the right colors, then."
Since the outer hull’s first layer was made of 64 light silver, Jin could alter its color at will. He worked quickly, finishing the exterior in a sleek pattern of black and matte silver-white.
"Elsa, would you do the honors of naming her?"
"Me!?"
"You named the Bernstein, didn't you?"
"...Mm." Elsa straightened her posture and faced the new submarine. Even in her everyday clothes, she looked solemn as she made the declaration. "I name you 'Shachi'."
Reiko, Edgar, and Jin all broke into applause.
"Let’s head out."
"Mm."
Jin took Elsa’s hand and led her through the side hatch of the Shachi. Reiko and Edgar followed closely behind. The entrance used a double-hatch system; the inner hatch would only open once the outer one was securely sealed—the same logic as a spacecraft’s airlock.
"Reiko, can I trust you with the helm?"
"Yes, Father. Leave everything to me!"
Like an airplane, a submarine moved through 3D maneuvering—though they didn't plan on performing many barrel rolls or loops—which could be disorienting for the uninitiated. Jin decided to leave the controls to Reiko, who had already familiarized herself with the vessel during the test runs. Reiko accepted the duty with visible delight. Once the autopilot’s control core had gained enough operational data, Jin intended to have it assist him so he could try piloting it himself.
"Here we go."
The dock gates opened, revealing the connection to the open sea. Using the Aqua Thruster, the Shachi began to glide forward.
"Oh, this is quite comfortable," Jin remarked. Their line of sight was nearly level with the water's surface, offering a fresh perspective as they left Tatsumi Bay behind.
"Now, we shall dive."
Reiko activated the gravity control magi-device. The Shachi became slightly denser than the surrounding water and began to sink slowly beneath the waves.
"Once we're cruising, we won't need to maintain this weight," Reiko explained.
The small fins on the sides—the main wings—and the fins at the rear—the tail wings—would generate hydrodynamic downforce during travel. This meant they didn't need to stay artificially heavy to remain submerged.
"Proceeding at normal cruising speed."
The Shachi surged forward under the power of the Aqua Thruster.
"How are we securing free ether underwater?" Elsa asked, voiced a sudden curiosity.
"It’s no different from being in the air, Elsa-san," Reiko replied.
Jin rubbed his chin thoughtfully. "I see. Since the particles of free ether are so incredibly small, they must not be hindered by the density of the atmosphere or the water. That suggests there might be plenty of free ether even in a vacuum."
Reiko’s voice cut through his musing. "Switching to force generator propulsion."
"O-Oh."
A moment later, a strange weightlessness washed over them. Because the force generator acted directly on the molecules of everything—including the ship and its passengers—the sensation of acceleration was non-existent.
"It’s... so fast."
Outside the window, the sea was a deep, clear blue. The transparency was high enough that they could see all the way to the seafloor, which was whipping past them at a staggering rate.
"Your speed is approximately 300 kilometers per hour," Laojun reported.
"I don't think it's safe to go any faster while Father and the others are on board," Reiko said, looking a bit apologetic.
In theory, the speed limit for generating wind and water-attribute barriers to shield the ship was roughly 3,600 kilometers per hour—Mach 3 in the air—meaning they could technically hit those speeds underwater. However, Laojun had been firm on the matter.
"I cannot recommend such speeds for a manned craft when considering the potential risks," he had cautioned.
"I understand," Jin said. "In that case, how deep can we actually go?"
The maximum depth for a modern submarine on Earth was typically 500 to 600 meters. Russian Sierra-class nuclear subs were rumored to reach 750 meters, though concrete data on military vessels was hard to come by. Some experts argued there was no reason to push a hull to its absolute limit, as it would simply implode instantly once its structural integrity failed.
"I believe 1,000 meters should be well within our reach..." Reiko said.
"Alright. Take us down slowly. I’ll use Craft Magic to monitor the hull for any signs of deformation. If things look dangerous, we’ll use the force generator to surface immediately."
"Are you sure?"
"Yeah. I think that’s the safest way to test it." Jin wanted to know their limits for the sake of future resource surveys.
"Elsa-san, just in case, please have the Warp Gate ready," Reiko added.
"Mm, roger."
The Shachi began its slow descent. They were currently near the edge of the continental shelf surrounding Hourai Island, where the 200-meter shallows dropped off into deep oceanic trenches. The water beneath them now was estimated to be 1,500 meters deep.
By using sonar to measure the distance to the seafloor and subtracting that from their starting depth, they tracked their progress.
"Depth 500... 550... 600."
They had officially surpassed the limits of conventional Earth submarines.
"700... 750... 800."
Outside, the world had turned pitch black. Jin focused entirely on monitoring the hull for stress.
"800... 850... 900."
Still no issues. The Gigant Peeder shell was proving to be even more resilient than he had hoped.
"950... 1,000."
As they descended, a Mermaid unit deployed by Laojun waited in the darkness. These specialized golems had already proven they could withstand pressures at 5,000 meters, and they kept a silent, watchful eye on the Shachi.
"1,100... 1,200..."
The Shachi’s powerful lights cut through the gloom, causing the seafloor to emerge as a ghostly landscape. White, snow-like particles drifted all around the vessel—"marine snow." Perhaps due to the deep-sea currents, the water's clarity shifted abruptly at this depth.
"...So beautiful," Elsa whispered. For a moment, the dangerous reality of the deep sea was forgotten in the face of the ethereal view.
"1,400. We have reached the seafloor."
The Shachi hovered just above the ocean floor. The hull deformation remained well within acceptable limits. Jin let out a long breath of relief.
"Grand Search."
Since they had made it all this way, he decided to scan the area for resources.
"Oh?"
"Jin-nii? Did you find something?"
"Yeah. There’s light silver and magi-crystal veins here. There are several other metal deposits quite close to the surface as well."
Laojun, listening in, immediately chimed in. "That is excellent news. I shall begin preparations for mining operations."
"Right. I’m counting on you."
It was a significant, unexpected bonus to their test run.
"Alright, there’s no need to push our luck any further today. Let’s surface."
They used the force generator to rise, returning to the surface in a single, smooth ascent. Because Jin had enchanted the interior with a barrier specialized in airtightness, they faced zero risk from pressure changes—a massive advantage of magical engineering. The magic maintained a constant internal pressure regardless of the depth.
When they finally broke through the surface, emerging from the dark abyss into the bright sunlight, even Jin felt a wave of relief alongside Elsa.
"A total success. Deep-sea resource gathering is going to be much easier now."
However, the trip had revealed one distinct flaw: humidity. The water vapor from Jin and Elsa’s breath had made the cabin uncomfortably damp. Condensation was forming on nearly every surface—a problem they hadn't noticed during Reiko’s solo test, as she didn't breathe.
"Looks like I’ll need to add a magi-formula for the water-attribute 'Condense' to keep the air dry," Jin noted.
On the bright side, the oxygen recycling system had performed perfectly. Jin had managed to build a magi-formula that monitored the nitrogen-to-oxygen ratio rather than simply breaking down all the CO2. It generated exactly enough oxygen to replace what they consumed, which was the most efficient approach. The separated carbon was being compressed into solid blocks of graphite.
"Maybe I can use that to make pencils..." Jin mused to himself.
Once they had ventilated the cabin and refreshed the air, Jin decided to spend a little more time enjoying the sea. He set the Shachi to cruise at a leisurely 50 kilometers per hour.
"Ah, this really is the best way to see things."
They watched as schools of colorful fish darted past their windows, losing themselves in the underwater scenery for a while longer.
And, of course, the very first thing Jin did upon their return was install a proper dehumidifier.