Watching the boat as it sank, I made a suggestion to Yuji.
"Hey, Yuji."
"Yeah?"
"Do you think we could use the hull of that boat sticking out of the water as a stepping stone to jump to the island?"
"Using that as a platform, huh? It looks a bit too far."
Taking my suggestion into account, Yuji compared the distance between the sunken boat and the island. After a moment of thought, he concluded it was impossible.
"I guess so..."
"The boat didn't even make it two meters from the shore. If it had reached the halfway point, we could have used the hull as a footing, but..."
The boat had gone down only two meters out. Our decision to test it near the bank for safety had backfired. Since it was two meters from the shore, that meant there was still an eight-meter gap to the island.
That was definitely not a distance we could clear while trying to pass as ordinary explorers.
"If only it had sunk a little further ahead..."
"No point in dwelling on it now. Let's find another way across."
"Yeah, you're right."
I cast one last lingering look at the sunken vessel before switching gears. I began searching for a method to reach the island that was within the capabilities of a normal explorer.
If I didn't have to maintain that cover, I would have just leaped to the island this very second.
"Even so, the way that boat went down... I wonder if they rigged the Kingston valve?"
"Probably. Considering how it flooded from the stern and dipped under, the drainage valve was likely tampered with so it would pop out easily."
There was probably a solenoid valve equipped with a timer and a water sensor. If so, the mechanism would activate the moment the boat touched the water. Depending on the timer's setting, they could make it sink right before it reached the island—or right after.
If that happened, even if a challenger reached the island safely, they would have no way to return without getting wet.
"They trick the challenger into thinking they can use the boat for a round trip, then leave them stranded once they're too close to turn back. That's a pretty nasty trap."
"Yeah. It’s one thing if you test it at the shore like we did, but if you’re short on time, most people would have no choice but to go for it. Then, while you're stuck on the island panicking about how to get back, the clock runs out. That’s likely the intended strategy."
"Which means there’s almost certainly something else in the pond. Something that would make anyone consider giving up the moment they fell in."
The trap-setter's intent was clear: they wanted to force the challengers into the water no matter what.
"Most likely. That makes it all the more important that we reach the island without setting foot in the pond."
"Agreed."
"Hmm..."
Yuji and I both racked our brains. There had to be some intended way to overcome this obstacle. But without equipment, we were stuck... wait.
"Maybe there was something useful among the gear they gave us at the gate."
"Oh, come to think of it, we did get those items. But they looked like they’d been tampered with. Using them might have just landed us in even more trouble."
"Maybe."
I thought back to the tools Staff-san had lent us at the entrance gate. They might have been rigged, but perhaps they were the key to this situation. For example, if we had rope, we could have tied branches together to make a simple raft...
As that thought crossed my mind, something caught my eye.
"Hey, Yuji."
"What? Did you come up with something?"
"Yeah. Think we can use... that?"
I pointed toward a specific spot. Yuji followed my finger.
"A cardboard box? I mean, we could probably fold it into something that floats, but it’s just paper. Even if it works for the trip there, it'll be a soggy mess by the time we need to come back."
"No, not the box itself—the contents. Those packing inserts inside... that's styrofoam, isn't it?"
"Styrofoam...! I see! You want to use them as floats!"
"Exactly. They won't sink even if they get wet, and depending on the size, they should be sturdy enough to act as temporary platforms for a jump."
It was the box that had held the assembly-style boat, so there had to be fairly large pieces of styrofoam used as cushioning.
"I see. Let's check it out."
"Yeah. We’ve wasted enough time already. Let's move."
Our investigations had eaten up more time than I’d realized; we only had about ten minutes left on the clock. Yuji and I hurried to the cardboard box and dragged it to the water's edge. At the bottom, we found nearly ten sturdy styrofoam boards, roughly forty centimeters square, each connected to a weight by a length of string. It seemed the boat had indeed been a loud, obvious decoy meant to distract challengers from the real solution.
"These are surprisingly tough."
I took a board in both hands and tried to bend it. It held firm without snapping. It would definitely support our weight for a quick jump.
"Hmm. With this much buoyancy, I can hop to the next one before it even starts to dip."
Yuji floated a board in the pond to test it. It stayed well above the surface.
"This'll work. Yuji, let's use these as stepping stones."
"Yeah. This is doable."
We began tossing the styrofoam boards into the pond one after another.
In less than a minute, we had created a path of floating islands spaced about one to two meters apart, stretching toward the center of the pond.
"Alright, we're set. Let's go, Yuji."
"Right behind you."
After checking our footing, we leaped out over the pond. The moment my foot hit a board, it began to sink under the impact, but I immediately kicked off and sprang to the next one. Yuji followed suit, hopping across the sequence of floats before they had a chance to submerge.
A few jumps later, we landed safely on the small island.
"We made it. That was actually pretty easy."
"Yeah. The return trip shouldn't be a problem either."
"Right. Now, let's grab that final item."
I walked toward the center of the island and found our prize. A key with the number '1' on it was encased in a transparent box. In front of the box sat a large cup, a small cup, and a basin.
Beside them was a wooden plaque with instructions.
"Let's see... 'Fill this basin with exactly four liters of water'?"
"The large cup holds five liters, and the small one holds three?"
"'You have only one attempt. Failure to produce the correct amount will result in immediate disqualification'..."
Yuji and I stared at the plaque in silence for a moment.
Then, simultaneously, we began tapping our temples and let out frustrated groans.
Why is there a math problem at the very end?!
What kind of trap even was this? What was the designer thinking, putting a logic puzzle here? I seriously questioned the sanity and aesthetic sense of whoever built this course.
Still, thinking about it calmly, it was a devious obstacle. The more you panicked, the harder it was to think clearly, and the pressure of a single mistake ending the run would paralyze most people. If you started second-guessing yourself, you’d waste precious time. While most people could solve it given enough time, doing it under the gun was another story. As a time-waster, it was incredibly effective.
"Hey, Yuji. Do you know how to measure out four liters with these?"
I picked up the cups and looked at Yuji. He was glaring at the wooden plaque, deep in thought.
"Give me a second. I think... if you use these two..."
"Wait, do you actually know how to do this?"
"Yeah, I'm pretty sure I saw a similar puzzle on a quiz show once... but the memory is a bit hazy."
"A quiz show... Oh! I remember that! It was ages ago!"
Yuji's comment jogged my memory.
"I’ve got it. You use the two cups to transfer water back and forth until you get the right amount."
"Transferring... Right, it’s coming back to me now."
Yuji looked away from the plaque, his expression clearing. We moved to the edge of the island to put the theory into practice.
"First, you fill the five-liter cup to the brim. Then you pour that into the three-liter cup."
Yuji dipped the large cup into the pond and filled it, then poured it into the small one.
"That leaves exactly two liters in the five-liter cup, right?"
"Yeah. Now, empty the three-liter cup and pour those two liters into it."
Yuji dumped the small cup and transferred the two liters from the large cup into it.
"And finally, fill the five-liter cup to the brim again. Then pour just enough into the three-liter cup to fill it up..."
"Since the three-liter cup already has two liters in it, it only takes one more. That leaves exactly four liters in the five-liter cup."
"Exactly."
Yuji held up the five-liter cup with a triumphant grin. I could understand the urge to celebrate, but we were out of time.
I gave him a quick nod of approval and nudged him back toward the center of the island. Once we poured the water into the basin, a clicking sound echoed, and the lid of the transparent box popped open.
"Done. That's all the items."
"Let's get to the goal. We're cutting it close."
"Yeah, let's move!"
I checked my watch as I pulled out the map. We had a little over five minutes left. This was going to be tight.
We hopped back across our styrofoam path and sprinted toward the goal. To save time, we took the shortest possible route—the one infested with the most traps.
When we reached the finish line, one final hurdle stood in our way.
"What is this now?"
"A door... with a ridiculous number of keyholes."
Blocking our path was a massive door featuring a seven-by-seven grid of keyholes. I tried the handle, but it wouldn't budge.
I pulled the keys we had collected from my pocket.
"I assume we use these, but..."
"We don't have time to try every single one."
"No kidding."
I checked my watch and sighed. We had just over two minutes left. With forty-nine keyholes and multiple keys, there were hundreds of combinations. We had to figure out the logic behind it immediately.
"There are numbers 1 through 7 and letters A through G labeled along the grid..."
"In a puzzle like this, the numbers 1 through 7 must correspond to the numbers on the keys."
"Most likely. The question is the letters. If we can figure out what A through G stands for..."
"Letters... wait. Letters? Hey, Daiki. Weren't there letters written on the edge of that map we got?"
"What!?"
At Yuji's suggestion, I scrambled to pull out the map. Sure enough, along the left margin, the letters A through G were printed in a vertical column.
"Look! If you look at the map coordinates, the spots where we found the items align with these letters!"
"You're right. So the letters on the door represent the locations."
Yuji looked over my shoulder at the map, nodding in agreement.
"Which means the number on the door is the key's number, and the letter is where we found it..."
"Daiki, try one. Use the key from the island in the hole that matches."
"Right. No sense standing around second-guessing ourselves."
I took the key we’d found on the island and inserted it into the 1G slot. I gave it a turn.
A heavy clunk echoed from inside the door as a lock disengaged. Our theory was right.
"It worked!"
"That's it! Come on, Daiki, we're almost out of time! Open the rest!"
"On it!"
I cross-referenced the map with the remaining keys, unlocking the mechanisms one by one. With only a minute left on the clock, the final lock clicked open. Together, Yuji and I pushed the door wide.
Phew. Somehow, we had cleared the Advanced Zone just in the nick of time.