“This thing’s really absorbed a lot.”

Shaking the water inside the test tube, Dewey muttered under his breath.

“Sorry. Klaha, can you pull up a little more water? I wanna check the boat’s durability first. If it’s mixed weirdly with the materials, we might capsize.”

“Got it. It’s better not to use magic to pull it up this time, right?”

“Yeah. Oh, maybe I should make something like a bucket. Do we have rope? Okay, just wait a sec for the bottle.”

As Dewey rustled through his belongings, Klaha’s gaze, almost unconsciously, was drawn toward the same place.

The waterway.

That dark current emerging from the depths of the ancient ruins—so black that even with magical illumination, its depths remained unfathomable. A flow of water leading to an unknown destination.

“Here, like this.”

“Much appreciated,” Dewey said as he handed it over. Klaha accepted it with a simple, “Thanks,” and tossed it in.

A thick, somewhat sticky splash echoed.

When she pulled it back up, the water inside the bottle was surprisingly clear.

“Thanks. I always end up breaking these against the water surface or the walls… wait, are there walls in there?”

“There should be… I think. I’ll check how spacious it is inside just in case.”

“For real? How can you tell?”

“I can’t actually see it, but I can get a rough sense of it through feel.”

“Huh,” Dewey said, impressed. With the bottle in one hand, he turned back to his belongings, while Klaha peered into the hole. Since using magic right away posed a risk, she picked up a nearby pebble and flicked it in.

The sound of the impact echoed.

It seemed… quite spacious.

“You can tell from that?”

“Ah!”

Startled, she let out an involuntary yelp. Someone had appeared right beside her, listening intently.

“…Sorry. Didn’t mean to scare you.”

“Ah, no, not at all!”

Before she could even regret her reaction, the person who had spoken—Ziel—was already silently regretting his own actions. Determined to shift the conversation, she pressed on.

“If it’s just the air pockets, I can get a rough sense of the space from the echoes—the distance of the returning sound, things like that. But when it comes to the waterway—”

“Even Eunice can’t tell, then?”

“Hearing that makes me want to compete, but… it’s like imagining the number of solutions to a higher-order equation. There are too many possibilities to narrow it down.”

“That’s just a thing for mages?” Wilae asked.

“…No?”

“Huh?”

Since the conversation was already continuing over there, Klaha kept silent. It seemed she and Ziel had spent enough time together to communicate with just a glance. He nodded, and they joined the others.

The team of mages analyzing the area, using the water they had drawn from below as their sample.

“Ah, Klaha. How’s the waterway?”

“Dewey is checking the boat’s durability now. As for the space, the width and length are fine, but the height might be an issue for transportation.”

“Eh? And that guy—what was his name again?”

“…Ziel.”

“Wait, Ney, you seriously never remembered Ziel’s name until now?”

“If he’s that strong, couldn’t he just manage without magic?”

“Well, I suppose,” Ziel said.

“Yeah, probably,” Klaha thought. But before she could analyze the potential issues that would arise from that, Eunice spoke up.

“No, the magic readings aren’t a problem, so we won’t need to rely on Ziel for transportation. I shook it around a bit, and at least near the entrance, it’s stable. The problem is—”

“Near the Mana Spot?”

“Yeah. Especially the new type of hotspot—we have no idea how it’ll mix or react. Unlike labyrinths, those are more unstable.”

Ziel, standing next to Klaha, looked confused.

To help him out, Klaha explained, “I think it’s permeating.”

“Permeating?”

She nodded. “I don’t know if it was originally a waterway, but right now, this underground passage is flooded. And that water has absorbed the magic leaking from the nearby hotspots. In other words, it’s become a channel for magic to flow through. Right, Eunice?”

Eunice, who had been studying the sample, twitched.

For a moment, Klaha thought she might have interrupted, but then Eunice turned slightly, his expression somewhere between troubled and amused.

With his thumb and index finger forming a circle, he made a gesture—“Correct.”

“So, does that mean this waterway itself is the source of the ‘tremors’? Like, I don’t know if this is way off, but could it be like built-up magic exploding?” Ziel asked.

Like gas, he added.

Klaha, as usual, was a little surprised.

She often felt this way when Ziel taught her swordplay. He had no background in magic—virtually none at all. Yet, sometimes, he showed a surprising level of understanding of magical phenomena.

At first, she figured it was just because he had traveled for so long. But sometimes, she wondered—could it be that he simply…

“Ziel really is sharp,” Eunice admitted. “That’s definitely a possibility.”

“…Thanks.”

Maybe Eunice was right after all.

“But I can’t say for sure. The magic might be building up like a gas pocket and causing explosions, but normally, unless there are factors like terrain, evaporation, or an influx of external liquids, magic tends to disperse evenly in water, making it stable. That’s why, for example, magic pharmacology—Roylen’s field—”

“Started with liquid potions, right? Though nowadays, with the church’s role, things have changed.”

Roylen glanced at Lililia.

They exchanged a warm, knowing smile, as if to say, “Thanks for your help.”

“So, while it’s hard to say for certain that this is the source of the ‘tremors,’ we at least know it’s the reason we’ve been stuck here.”

“So what’s the next step?”

“We take a look. See where it leads.”

That’s the fastest way, Eunice said.

“We could recalculate the propagation model, but in the end, to be certain, we’d need more data. And since we’ve found a suspicious spot, there’s no harm in investigating.”

With a flourish, he added, “Sometimes, even mages need to leave their desks and check things out firsthand.”

“Mages do that?” Wilae asked.

“That part’s true,” Lililia replied.

Dewey, who had been working on the boat, suddenly returned, along with Ney.

“No issues here. The countermeasures for the Mana Spots are working. No chance of capsizing the moment we touch the water.”

With that, Roylen, as the lead investigator, took over.

“As Eunice suggested, investigating the waterway is our best option. The question is, who goes?”

“Pass.”

“Pass.”

Dewey and Ney raised their hands immediately. Roylen chuckled.

“What do you think, Klaha?”

She calmly responded, “Just like when we entered the ruins, we should split into an advance team and a standby team.”

She made the same proposal as always.

Inside an old underground structure, there is a high possibility of encountering geographical troubles such as collapses. The worst-case scenario would be being unable to bring back the new information obtained here. Therefore, even if the additional investigation ahead fails, we should leave enough manpower in the standby group to ensure a safe return to the research facility. With that in mind...

“Roylen, Wilae, Ney. If we assume daytime activity, then Eunice as well. And one of us should remain as a guide for the return journey. I believe one of us should be in the standby group.”

“Oh, this is starting to feel like a river-crossing puzzle.”

Eunice muttered “river-crossing puzzle” in a small voice. Dewey responded with, “You seem like the type to enjoy puzzles.” Klaha chuckled and added,

“It shouldn’t be that complicated. And then there’s the matter of combat strength. Since we are relatively close to the research facility and have been clearing the area thoroughly, the person chosen as the guide—aside from Ney and myself—should also be able to serve as combat support. Also, Ziel or Lililia. Naturally, both of you are strong candidates for combat support.”

“And here stands a lonely man who is neither useful for combat nor as a guide…”

“No, Dewey is definitely in the standby group.”

Roylen followed up on Klaha’s statement.

“What the standby group needs is the ability to return without the advance group if necessary. You are essential.”

“Why?”

“Because the boat will go with the advance team. Otherwise, we’d have no way back.”

“...Oh!”

Dewey’s eyes widened. “Wait, does that mean if things go south, I’ll have to build a new boat right here?” Ney jabbed him in the ribs. “That’s your job, isn’t it?” He retorted, “That’s outside my official duties.”

“Or I could stay behind. It would take some time, but the success rate of our return would be much higher.”

“No, I’d prefer to have Lililia in the advance team.”

Finally, Ziel spoke up.

“Since this is a prehistoric ruin, there’s a possibility it’s connected to the Apocrypha. In that case, I want to be as prepared as possible. Lililia and Eunice as well… Ah, I was assuming I’d be in the advance team.”

“No, having you in the advance team would be a huge help, Ziel. Among all of us, you have the highest combat ability, survival skills, and adaptability.”

With that settled, Roylen continued,

“Then, let’s confirm Ziel, Lililia, and Eunice as part of the advance team.”

“That just leaves the guide. I’ve already been given the noble duty of waiting, so Ney, it’s your turn.”

“No way.”

“Personally, I would like to go.”

Roylen glanced at Wilae.

“Ideally, the older ones should take on the dangerous tasks…”

She raised both hands near her shoulders.

“But I wouldn’t want to let my sense of adventure die because of that. I’ll stay in the standby group. The rest of you can decide among yourselves.”

“Looks like we have a final showdown, Klaha.”

“Huh?”

She hadn’t said a word, but was it that obvious? Roylen turned to her with a knowing expression, as if to say, “Of course, you want to go forward, don’t you?”

“Let’s settle this fairly. Rock-paper-scissors.”

“Ah, if Roylen really wants to go—”

“Rock, paper, scissors—”

Pon.

“Would you say your unshakable resolve led to victory, Klaha?”

“Yes…”

The waterway they traveled through during Lililia’s victorious interview was unbelievably dark. The light Eunice had carefully cast for stability only illuminated a small area around their fingertips. The only sounds were their voices and the flowing water. Nothing else.

Plop. Plop. Ziel’s oars made a heavy sound as they moved through the water.

Seated on the boat, Klaha felt like they were gathered around a campfire at night.

“I do feel bad about it, though. Roylen has been investigating the southern Sea of Trees for a long time.”

“…N-no. I don’t think you need to feel that way. Professor Roylen is primarily interested in pharmacology. He mentioned before that he doesn’t have a particularly strong attachment to the Sea of Trees itself.”

“Really?”

“Yeah. I mean, he’s been researching it for a long time, so I’m sure he has some attachment. But it’s not something you need to worry about…”

Klaha smiled at Eunice, appreciating his consideration. Then she realized the darkness might obscure her expression, so she added a small, “Thank you.” Her voice echoed through the vast cavern, making her feel slightly embarrassed.

“But don’t you think all mages throw scissors first in rock-paper-scissors?”

Suddenly, Lililia made a bold claim.

“Huh? Where does that bias come from?”

“And clerics always start with paper. It reflects their desire to embrace everything with an open heart.”

“I distinctly remember losing rock-paper-scissors to you once.”

“That’s because paper is the strongest move.”

“That ruins the whole game.”

Just as Klaha’s embarrassment started to fade in their lighthearted conversation—

“You okay?”

Ziel’s quiet voice reached her.

“The path. I’m sorry for relying on you completely.”

“It’s fine. We’ve just been going straight so far. As long as I keep track of the distance, we’ll be fine.”

“We’ve been going straight the whole time?”

“Huh?”

“Huh?”

“Ziel, are you enjoying planting seeds of doubt in Klaha’s mind?”

“Okay, I’ll shut up now.”

There was no need for him to stay silent, but Lililia was right—it had made Klaha anxious. She felt like they had been moving in a perfectly straight line, but when she turned around, she saw nothing but darkness. The kind of darkness that made one question whether reality even existed.

“I’m glad Klaha came with us.”

Eunice murmured softly.

“I’m used to the dark, but I still struggle indoors. Looking from the outside is fine, but once I step inside, I completely lose my sense of direction.”

Klaha was secretly, intensely—deeply moved by those words. But precisely because of that, she resolved to do her job properly.

“But I wonder how far this goes.”

She voiced a constructive thought.

“If this waterway extends throughout the entire Sea of Trees, it might take more than a day to reach our destination.”

“Yeah, and if we take too long, we’ll run out of food. Eunice, do you think there are fish in here?”

“Hmm… If this water has always been here, probably not. But if it flows in from somewhere, there might be some.”

“Then maybe someone should fish while I row. Water alone will only keep us alive for about three months.”

“You’d be dead.”

“I could probably last a year.”

“Huh…? Am I in the minority for thinking that’s impossible?”

Ziel glanced over at Eunice, who met his gaze with a firm nod. While internal energy and holy magic could provide some level of deception, the general understanding was that one could survive on water alone for about a month at most.

"Right?" Eunice said in relief, but then added, "But still—"

"I don’t plan on taking that long to begin with. Let’s speed things up. Lililia, can you cast holy magic on Ziel?"

"Oh, is that okay?"

"For now, holy magic doesn’t seem to be an issue. Unlike other types of magic, it’s much more stable and doesn’t interfere with its surroundings. If anything bothers me, it’s that despite the amount of magical energy here, there’s no faint glow like inside a labyrinth. But if I can analyze that a little further, I’ll join in on the acceleration too."

It seemed like he had been analyzing their surroundings, even though it looked like he wasn’t doing anything—or rather, it was simply too dark to see him.

"Spending days on a boat this small together isn’t exactly a pleasant thought, right, Ziel?"

"...Well, to be honest, it’s exhausting having to be considerate all the time. You probably hate having me here too."

"No need to cry about it. I can smooth things over."

"I’m not crying—"

"To be honest, I also... no, to put it plainly, I really value having my own personal space. So this situation is quite uncomfortable for me."

With that, he continued,

"Let’s just get this over with. And if my guess is right—this is a ‘station.’"

If that were the case, the travel time shouldn’t be too long.

In the end, it only took a few hours before they actually found the location.

"What... is this?"

At the end of the waterway, a waterfall came into view.

They stopped rowing. Ziel rolled his shoulders, but thanks to Lililia’s powerful holy magic, neither stiffness nor fatigue remained.

Now, the area around the boat was illuminated by a light that Eunice had deemed safe to use.

Gray, earth-toned inorganic walls. A filthy ceiling. Water thickened by absorbed magic. The bottom of the waterway was clouded by soil and sand carried in from somewhere, making it impossible to see.

But in front of them, it was clear.

The roaring sound of water crashing down.

A thick tunnel, reinforced with some unfamiliar material, serving as the water’s passage.

"Looks like we hit the mark. This seems to be the source, right?"

"Yeah, seems like it."

Relieved, Ziel agreed with Lililia. Just as Eunice had said earlier—wandering inside a labyrinth with only three people was stressful enough, and traveling with Klaha also required a lot of effort. But being stuck together on such a small boat? That was on another level entirely.

At least now, they didn’t have to worry about that.

They had found what seemed to be the right place.

"Something feels off."

"Huh?"

But Eunice muttered something unexpected.

"I thought it was a station, but maybe it’s actually a drainage tunnel? If that’s the case, the scattered ruins don’t make much sense."

"Still, there’s got to be something up ahead, right?"

"Oh, yeah, that’s true. We should investigate the surroundings a bit more. If it looks safe, we could try finding a place to go ashore."

Ziel sighed in relief. "Makes sense."

Eunice continued his deductions.

"Hmm...? Or maybe they combined a transport route with a drainage system? That doesn’t seem likely, though..."

"Are we even sure this is drainage? Couldn’t it just be that the changing terrain accidentally connected it to the sea?"

"If that were the case, the salinity would be higher. At the entrance, I thought the water was just rainwater accumulation."

"Ah... Then maybe the partition between the transport and drainage sections deteriorated over time? Can you see over there?"

"Hmm?"

"There are marks showing the water level once reached the ceiling. Maybe the drainage system failed, flooding the section completely. That could have broken the partition and let the water into the transport section."

"Oh... that makes sense. At first, the lighting wasn’t good enough to see properly, so we might’ve missed signs of a breach. Klaha, how’s the route looking?"

"I’ve been able to observe the surroundings since a while ago, and I’ve mapped the path. Compared to our usual exploration routes, this one is extremely linear. Not just in terms of speed, but the overall travel distance is much shorter."

"That helps a lot," Eunice said.

Klaha smiled. "I’m glad."

Ziel, seeing how much they had warmed up to each other, gave a small nod.

"So, what do we do? Should we try going back through this drainage tunnel?"

He made a proposal.

"Go... back? Wait, could we actually do that?"

"I think we could swim against the current if it’s just this much."

"You’re like a salmon, Ziel," Lililia teased.

"I’ve beaten a salmon before."

"Wow."

"But yeah, one person should be able to fit inside this tunnel. Hold on. Let me check if the water is harmful to humans."

Klaha immediately took a bottle and collected a water sample. Eunice received it and examined it.

"...Yeah. It seems fine for now. But the fact that it already carries some magical energy is concerning."

"If it was adjacent to a transport route, it shouldn’t be too hazardous, considering sanitation. But I’m more worried about the condition of the drainage source."

"Right. If any processing was involved, this could be connected to a treatment facility. Hmm... This could be dangerous."

"We could go a little ways in and turn back if it seems bad?"

"Wait, do you think you can actually just go in a little and come back?"

“…If it’s a straight path.”

“It’s a gamble…”

“W-wait a moment.”

With a rustling sound, Klaha once again rummaged through her luggage—what exactly did she keep in there?—and then,

“I think it might make swimming a little harder, but how about tying this rope to your body somewhere?”

She offered a great idea.

“That’s a good one. If we follow the rope, we can find our way back.”

“Yes. I’ve ensured it’s long enough, but if it reaches its limit, that will also signal the need to turn back.”

“Right. If we try to go as far as we can while we can still hold our breath, there’d be no end to it.”

“What do you mean? There is an end—your breath runs out.”

“Have you ever beaten a whale?”

“I’ve kept pace with one.”

Ziel took the rope with a determined nod.

Despite his bravado, he couldn’t tie it himself, so Klaha helped fasten it around his torso. If you reach the other side safely, pull the rope three times, she instructed. He gave a simple nod in response.

Then, Lililia added,

“Let’s take some extra time to put up a protective barrier. It’d be awful if you got ambushed again.”

She carefully cast holy magic.

Five minutes, then ten. She held her hands over his chest, and a gentle warmth spread through his body.

“With this, you should be able to withstand about three blasts of that attack without a scratch,” she said, lowering her hands.

Her words were surprising.

“Impressive. Your skills have improved again?”

“That’s part of it, but holy magic isn’t an instant-cast type. The more time you put into it, the stronger it becomes. It just didn’t sync well the last time because, for Ziel, applying too much magic actually hinders his movement.”

Ziel flexed his arms to test it.

There was a slight stiffness in his movements, but compared to his time in the highest-difficulty labyrinth, it was a major improvement. If he could take three hits unharmed, the benefits outweighed the slight decrease in agility.

“I also set up a light, and it’ll react to toxins, just in case. If the water turns dangerous, it’ll be quite a spectacle, so come back if that happens.”

“A spectacle…?”

“Look forward to it.”

It sounded both terrifying and genuinely exciting.

With that, all preparations were complete. Ziel twisted his waist from side to side, then opened and closed his shoulder blades.

“Alright, I’m going.”

With three voices telling him to be careful at his back—

He leapt toward the waterfall with all his might.

The drainage pipe was wide enough to swallow multiple people with ease.

The water flow was as fierce as a downpour in the Sea of Trees, but Ziel swam through it swiftly, like a spear.

He paddled with his hands, kicked with his feet. The movements weren’t difficult. He surged forward, pushing against the current. Then, midway, he realized something—the strength of the flow varied depending on the location. Sometimes it weakened, other times, for a brief moment, it intensified.

Perhaps it wasn’t a single straight path. There might have been multiple sources feeding in, or multiple exits leading out.

But as long as he had the rope, he could always return.

Relying on the lifeline tied around his torso, he focused solely on swimming against the flow, following the path ahead.

To Ziel, it felt like he had covered a vast distance.

With no companions to pace himself with, he moved as fast as he could. He felt as if he had circled a small city five times over. But considering the actual length of the rope, that was impossible. He inwardly scoffed at how unreliable his own sense of distance was.

Even so, he eventually arrived.

A vast, open space.

“…?”

He didn’t think it was the sea.

His vision suddenly opened up, and he realized he was at the bottom of something. Some kind of bottom. He knew because he had emerged from the drainage pipe and also because the light had increased.

It wasn’t the glow from Lililia’s magic, which had been set to mark the rope. This was something else. Strangely bright. Not the sea—because the sea’s depths weren’t this well-lit. There were no fish. Nothing at all.

Where exactly was he?

To find the answer, he swam upward.

Breaking the surface, he took a breath.

“…A water tank?”

Strangely enormous.

Large enough to house multiple whales with room to spare.

Ziel stood there, floating in a water tank illuminated by an eerie blue-white artificial glow from below.