On December 19, Jin and Erza arrived at the dock of Tatsumi Bay after breakfast.

They were finally going to board a submarine to explore the bottom of the ocean.

"Brother Jin, what is the name of this boat?"

Erza notices just before he asks.

"Hmmm, I see. We need a proper name. is just what it is, and < is not good either. ......

Hitoshi is still troubled.

"Hmm, how about ......<>?"

"...... is a good idea."

"Okay, let's make it the right color."

The first layer of the outer panel is 64 light silver, so the color can be changed freely. Jin quickly finished it in black and white (actually matte silver-white).

"Elsa, name it for him."

"What, me?

Bernstein was Elsa, too.

"......"

With her clothes still on, Elsa straightens up, turns to the new submarine and declares.

"I name thee ."

Reiko, Edgar, and Jin clapped their hands.

"Come on, let's go."

"Yes."

Jin took Elsa's hand and climbed through the hatch on the port side onto the . Reiko and Edgar followed.

The entrance and exit is double, with the inner hatch opening when the outer hatch is closed tightly.

It is the same concept as the airlock on the spaceship.

"Reiko, can I ask you to take the controls?"

"Yes, Father. Leave it to me!

Submarines, like airplanes, do not do much three-dimensional maneuvers ...... such as kirimomi and somersaults, but they are ...... confusing if you are not used to them.

This time, Jin decided to let Reiko, who was used to test navigation, pilot the submarine. Reiko is happy to be in charge.

When the control core for the autopilot is sufficiently advanced, Jin plans to try piloting the ship by himself with the help of Reiko.

"Okay, let's go.

The dock gate opens, connecting to the sea. First, using the Aqua Thruster, <> begins to move slowly.

"Oh, it's quite comfortable.

It is also refreshing to be so close to the surface of the sea.

Meanwhile, the orca has left Tatsumi Bay.

"Well, let's dive in."

Reiko activates the gravity-control spellcaster. The orca, now slightly heavier than the water, slowly sinks below the surface.

"I don't need it anymore to navigate underwater."

The small fins ...... on each side of the main wings and the fins ...... on the tail fins behind them generate hydrodynamic downforce while sailing, so there's no need to go too heavy. So there is no need to be heavy.

"First, normal navigation."

moving forward with aqua thrusters.

"How are we going to secure free magic element (ether) underwater?"

Erza suddenly asked the question that had just occurred to her.

"Yes, Miss Elsa. It is the same as in the air.

Hearing this, Jin pondered.

"Well, the particles of free magical element (ether) are so small that they are not affected by the air or water, aren't they? If ...... is the case, there is a possibility that there are enough free magical elements (ether) in a vacuum.

Reiko's voice broke through this reverie.

"Then let's propel the force field generator!"

"Oh, oh."

A beat later, a floating sensation hits us. The force field generator works directly on the molecules of everything, including the entire ship's ...... crew, so they will not feel the acceleration.

"Ha, fast."

Outside the window is blue water, but the transparency of the water allows me to see the bottom of the ocean, which is passing behind us at great speed.

The old man informs me.

"It's not safe to go any further with your father and the others on board.

Reiko says with an apologetic look on her face.

Actually, 3600 km/h, or Mach 3 in air, is the upper limit of the speed at which the wind and water shields are generated, so theoretically, the boat should be able to reach 3,600 km/h even in water. In theory, we should be able to reach 3,600 km/h underwater.

The old man also nailed me with a nail.

"All right, then. Then, what is the maximum submarine depth?"

The maximum submarine depth on Earth today is about 500 to 600 meters. The Russian Sierra nuclear submarine is said to be around 750 meters (no definite data is available since it is for military use).

Some experts say that the hull of a submarine is at risk of crushing at once when the pressure resistance exceeds the limit, so they do not bother to check the limit of the hull.

"I think we can get to 1,000 meters. ......"

"Okay, let it sink slowly. I'll use my engineering magic to monitor the hull distortion. As soon as the ship is in danger, use the force field generator to levitate it.

"Are you sure?"

"Yes, I think it's less dangerous that way."

Jin wanted to know the maximum dive depth for undersea exploration.

"Erza-san, please prepare the warp gate just in case.

"Yes, sir."

Thus, the orca slowly sinks.

In fact, the area where Horai Island is located is where the 200 meter deep gently sloping land called in topography ends, and there is a so-called that plunges deep at once in the vicinity.

The area where Jin and the others were currently located was estimated to be about 1,500 meters deep.

Using sonar, they calculate the depth by subtracting the distance from their current position to the seafloor from the depth measured at the sea surface.

"......Depth500......55 0......600"

The submarine depth of a modern Earth submarine is exceeded.

"......700......750......800"

It is now pitch black outside. Jin concentrated, monitoring the distortion of the hull.

"......800......850......900"

Still no problem. The shell of the gigantopod seems to be better than expected.

"......950......1000"

As a matter of fact, the mermaid squads under your direction are standing by outside. They have a track record of withstanding water pressure of 5,000 meters, and they were quietly watching over the orca.

"1100......1200......"

The bottom of the ocean floor appeared dimly under the powerful lights.

Around the orca was a white, snow-like floating mass. <. The transparency of the water changes drastically around this point, probably due to the ocean currents.

"...... clean"

The view made me forget for a moment that I was in the dangerous deep sea.

"......1400, the bottom of the sea."

< was a 1400 meter seafloor. The distortion of the outer shell was still acceptable.

Jin let out a sigh of relief.

"<>"

Since he had just arrived at the seafloor, Jin checks out the undersea resources just in case.

"What?"

Jin, what's wrong?

"Oh. ...... There are light silver and magi crystals. There are also veins of various other metals in quite shallow places.

The old man heard these words.

"Yes, please."

The unexpected harvest came as an added bonus.

"Okay, there's no point in pushing ourselves any further. Let's surface."

To surface, they use the force field generator to return to the surface in a single bound.

Since the hull is covered with airtight wards specialized for the wind attribute, there is no risk of pressurization or depressurization, which is a great advantage.

It has no effect on magic and keeps the atmospheric pressure constant.

Not only Erza but also Jin was relieved when they emerged from the pitch-darkness of the deep sea to the bright surface of the sea.

"This is a great success. This will make the search for undersea resources easier.

However, there was one drawback that was revealed.

It was humidity. The water vapor contained in the exhalations of both Jin and Elsa made the cabin damp ...... or, to put it more simply, there was condensation everywhere.

This is a flaw that Reiko, who does not breathe, could not see when she tested.

"This means we need to use water magic < to remove the moisture. ......

In contrast, the recycling of oxygen worked well.

This is because Jin was able to construct a magic formula (Magi Formula) that determines the ratio of nitrogen to oxygen, instead of decomposing all the carbon dioxide in the air.

In other words, the control method of producing only the amount of oxygen used was the correct one.

The separated carbon is solidified as graphite.

"I should make a pencil ......", Jin thought.

Jin was thinking "I should make a pencil.

I'm going to stop talking now.

After surfacing and changing the air, Jin started running again to enjoy the underwater world. This time at a speed of about 50 km/h.

"Ah, this is just about right," he said.

They could see colorful fish swimming in the water.

Jin and Elsa enjoyed the underwater scenery for a while.

Needless to say, they took measures against humidity after returning home.