Chen Hao stuffed the last piece of rope into the side pocket of his backpack and clapped his hands. The raft, tied to the rocky edge, swayed gently with the current, like a dog waiting to be woken up.

He stared at the water for two seconds, then suddenly grinned: "Alright, Desolate Star One, don't let my worn-out hands go to waste."

He bent down, grabbed the rope, and pulled hard. The knot loosened, and the log slid into the water, its front end sinking slightly, splashing water in a circle, before finally stabilizing.

"Let's go," he said, stepping onto the lap.

The raft swayed slightly, but he steadied himself, arms outstretched for balance, like a student giving a speech for the first time. Nana followed closely behind, her steps so light they barely caused a ripple. She stood at the back, her robotic arm slightly extended, ready to respond to any unexpected situation.

“You’re standing like a riot police officer.” Chen Hao turned around and glanced at her. “Can you relax a little? This is a rafting trip, not a bomb defusal trip.”

“The current environment doesn’t have an entertainment aspect,” she said. “I suggest you sit down first.”

“Sit? That’s so unimpressive.” He said, but still slowly squatted down. As soon as his bottom touched the plank, the raft swayed again, and he immediately reached out to hold onto the crossbeam next to him. “Okay, I’ll sit.”

The current propelled them forward slowly, at first almost imperceptible, with only the rocks on the bank receding little by little. The cave ceiling was so high that its outline was invisible, with only scattered moss emitting a faint light, as if someone had casually sprinkled some fluorescent powder.

"You're asking if anyone has been here before us?" Chen Hao looked up. "I mean, before us."

"Based on geological data, the cave was formed more than 100,000 years ago," Nana said. "Human civilization has existed for less than 20,000 years. From a probability standpoint, the possibility is extremely low."

"Couldn't you just say 'no one should have been here'? Why do you have to calculate it to be 100,000 years?"

"I am stating the facts."

“The truth is often painful.” He sighed and reached out to touch the piece of wood at his feet. “But it’s also pretty cool. We might be the first people to float on this river.”

"It might be the last one."

"Huh? Why does that sound a bit depressing?"

“I’m just stating the logical conclusion.” She paused. “If the subsequent shipping lanes are unstable, we may not be able to return.”

"Look, here we go again." Chen Hao waved his hand. "Can't we let hope live a little longer?"

The riverbed gradually widens ahead, and more and more stone pillars hang from the rock walls on both sides, some reaching the water's surface like strings of a musical instrument submerged in water. The water flow creates subtle vibrations, producing a low-frequency hum that, mixed with echoes, vibrates back and forth within the cave.

Chen Hao listened intently, then casually pulled a flat stone from his bag and tapped it on the water's surface.

"Thump."

The sound wasn't loud, but the entire space seemed to respond.

He tried again, this time changing his position. "Thump, thump, thump," the rhythm came through, and he laughed at himself first.

"I'm keeping the beat." He turned his head. "Want to join me for a bit?"

“I don’t have a music module,” Nana said, “but I can report the water flow speed: 0.6 meters per second, the channel width is maintained at five meters, and there are no branches.”

“You’re a real romantic killer.” He put away the stone shard, leaned over the edge of the raft, and looked down at the water. It was so black that it was impossible to see the bottom, but the surface shimmered with tiny reflections, like it was sprinkled with silver powder. He reached in and it was chillingly cold.

"How's the water quality? Can I take a shower?"

"The sulfur content is too high, and long-term exposure may cause skin irritation," she said. "Furthermore, the current environment is not suitable for undressing."

"Tsk, even robots meddle in other people's business."

"I'm just preventing you from doing anything that would lower your survival rate."

He withdrew his hand, shook off the water droplets, and suddenly looked up: "Wait...did you hear me?"

Nana flashed a blue light and remained silent for two seconds.

“The sound of the flowing water has changed,” she said. “The frequency has increased, which suggests that the slope ahead is getting steeper.”

Before he could finish speaking, the front of the raft suddenly sank.

Chen Hao lunged forward, nearly plunging into the water, and hurriedly grabbed onto the central beam. The stern of the raft lifted up, and Nana quickly lowered her center of gravity, placing one hand on his waist and forcefully pressing him back into place.

"Don't move," she said.

Within seconds, the raft changed from a gentle slope to a tilt, the current noticeably quickened, and a roaring sound gradually rose in our ears, like an old-fashioned washing machine turning on in the distance.

"This is really turning into a rafting trip!" Chen Hao gritted his teeth. "Can it even stop?"

"No effective braking means." Nana scanned ahead. "We expect to enter the acceleration phase in thirty seconds. We recommend keeping your body still."

"Secure my foot! Where's the seatbelt?"

"The third rope loop on your left is a reserved fixing point."

He turned his head and saw a thickened knot embedded in the edge. He quickly reached out and grabbed it, and the raft lurched again, as if stepping onto an invisible step, and its speed increased dramatically.

Water began to splash upwards, wetting his trouser legs. The light from the cave ceiling was dwindling, replaced by a faint white noise coming from ahead—the sound of water crashing against the rocks.

“Do you remember when we were building this raft, you said ‘prevention is better than repair’?” he shouted.

“I remember,” Nana replied, while slightly spreading her legs to increase her support.

“Now I think,” he swallowed, “we might have overlooked a crucial point—preventing the boat from capsizing!”

The raft swayed back and forth in a murky current, one side almost touching the surface. Chen Hao gripped the rope ring tightly, his other hand instinctively reaching for Nana's arm.

Instead of dodging, she pulled him closer to the center.

"Focus your center of gravity," she said. "Don't fight the current, go with it."

"Easy for you to say... You're not going to drown!"

"I'll short-circuit."

"Isn't that the same thing!"

The light ahead suddenly brightened, not from overhead, but from a sliver of ethereal blue seeping through a crevice in the rocks to the side. The light reflected on the water's surface, torn into ripples by the flowing current, like liquid neon.

Chen Hao was stunned for a moment: "This place... actually has a light source?"

“It’s probably a mineral precipitation reaction.” Nana scanned quickly. “The glowing area is concentrated in the right front crack, lasting for about seventeen minutes, and appears periodically.”

"So, we got to see the live broadcast?"

"I guess so."

He grinned, about to say something, when the raft suddenly crashed into an invisible water step.

The entire structure shook violently, its front end plunging directly into the water, splashing up a large spray. Chen Hao was thrown forward, only managing to stay upright thanks to the force of his arms. Nana knelt on one knee on the raft, her mechanical arm wedged into the seam, using her own weight to hold down a corner that was about to lift up.

"It held up!" he gasped. "This thing hasn't fallen apart yet!"

“The node’s capacity is still within the threshold,” Nana said. “But we suggest reducing verbal output to conserve energy.”

"You're really nosy," he cursed, but obediently shrank back and lay down again.

The current continued to accelerate, and the roar grew louder and louder. Horizontal grooves began to appear on the cave walls, as if they had been eroded by some enormous object over a long period of time. Occasionally, small rocks would fall from above, hitting the raft with a dull thud.

"Do you think there might be a waterfall ahead?" he asked.

“Based on the terrain slope, the drop is insufficient to form a vertical waterfall,” she said. “However, there may be continuous rapids.”

"So, we have to keep riding this water roller coaster?"

"Yes."

"I should have put a steering wheel on the raft," he muttered, rubbing his aching arms. "Or at least a lifebuoy."

"You have a spare flotation device in your backpack."

"Oh right." He flipped through them. "The one with a plastic bag and three batteries bundled together?"

"That's an emergency signal device."

"I knew it."

They continued downstream, the raft bobbing up and down in the turbulent current, but it never broke apart. Chen Hao gradually adapted to the swaying, and could even look up to observe his surroundings during the lulls in the swaying. The structure of the cave ceiling became even more bizarre; some places looked as if they had been melted by high temperatures, leaving behind a smooth, glass-like surface that reflected scattered blue light.

"Are these rocks the ones that were burned by lava before?" he asked, pointing to a dark red rock layer.

“It’s possible,” Nana said. “The area has experienced several periods of crustal activity.”

"Could the river we're floating on now be a drainage channel from an ancient volcanic crater?"

"The speculation is reasonable."

"It's kind of cool to think about," he chuckled. "The two of us sitting on a handmade raft, floating inside the volcano of a dead planet."

“The analogy is inaccurate,” she said. “This is not intestines.”

"You insist on being so serious."

The sound of water grew louder ahead, and the passage seemed to be narrowing. Nana's blue light continued to flash as she constantly adjusted her posture to cope with the changing water flow.

Chen Hao took a deep breath and gripped the rope ring tightly again.

“Come on,” he said, “let’s see where this wretched river is trying to take us.”