The seedling trembled again.
Chen Hao stared at the leaf, his finger still hovering in mid-air. The tremor just now was more pronounced than last time, as if someone had gently flicked a fingernail through the glass.
"Nana." He didn't turn around. "Don't bump into the wall."
She paused when she heard this, her blue light swirling in her eyes before refocusing on him. Her palm was only two fingers' distance from the rock face.
"You saw it?" she asked.
“It’s not about seeing, it’s about feeling.” He squatted down, hands on his knees, “It’s like… before you step on that loose piece of wood in the floor, your feet know it’s going to collapse.”
Ten seconds passed, and the rock face remained still, not even the traces of the water droplets had changed. Yet there was an inexplicable tension in the air, like a damp towel that had been left out to dry for too long, as if a single fiber might snap at any moment.
Nana pulled up the data stream: "In the past ten minutes, the rock strata have experienced three low-frequency fluctuations, with intervals of about three minutes, and the amplitude has been increasing. The frequency is 0.8 Hz, which does not conform to the settlement model."
"What does that mean?"
“It doesn’t look like the stone cracked on its own.” She put away the projector. “It’s more like—something is moving inside.”
Chen Hao grinned: "Like water?"
"There is a 37% chance. It could also be due to gas release or biological activity."
“A living thing?” His eyebrows twitched. “Don’t tell me there’s an organ growing on the wall.”
“There are no signs of life at the moment.” She turned to the planting area, “but if there are hidden waterways, it could affect the stability of subsequent irrigation.”
Chen Hao stood up and dusted off his pants. "Alright, since it's not going to stay put, we can't just sit here and get soaked." He picked up a metal rod leaning against the stone platform. "Instead of waiting for it to leak, let's find the source first."
"You've decided to venture into unexplored territory?"
"What else? Expect it to write itself a letter saying, 'Dear residents, I will be spraying water from behind the third brick on the east wall at eight o'clock tonight'?" He shook his head. "We've only gone less than halfway through the hole. Who knows if there's a bigger hall, better lighting, or even...clean water in the back?"
Nana paused for two seconds: "Risk assessment: Unknown terrain, unstable structure, and increased probability of communication disruptions."
“I know,” he said, slung the stick over his shoulder, “but do you know what the worst is?”
Please explain.
“The worst thing isn’t dying out there,” he grinned. “The worst thing is dying of thirst five meters from a river when you could have lived, but you were too lazy to move—that would be so unjust.”
The blue light in her eyes flickered, as if a small bug had occurred in the program.
“The logic holds,” she said. “Initiate the expedition protocol.”
The passage was much narrower than before, with stalactites hanging low overhead, forcing Chen Hao to walk with his back hunched. His chubby body was squeezed into the crevices of the rocks, like a sausage stuffed into a funnel. At one point, the ground collapsed, revealing a dark pit that seemed bottomless. The wind blew up from below, carrying the smell of rust mixed with damp earth.
“If we fell down,” he said, moving closer to the rock face, “wouldn’t that be considered a work-related injury?”
"Without an employer-employee relationship, the definition of work-related injury does not apply."
"Sigh, I just wanted some peace of mind."
Nana walked ahead, arms outstretched for balance, the magnetic attraction system under her feet humming softly. She suddenly raised her hand to signal to stop.
"The slope ahead is 65 degrees. We recommend using the assisted climbing mode."
"You could have just said I couldn't climb up." He took a breath. "Come on, lend a hand."
She turned around, reached out and pulled, and Chen Hao was lifted into the air for half a second, almost stepping on his own shoelaces when he landed.
"Be gentler next time, my internal organs haven't adjusted to weightlessness yet."
"The intensity has been adjusted."
The deeper they went, the more humid the air became, and the headlamp began to flicker, as if the battery was running low or the fog was fogging up. Chen Hao tapped the lamp casing twice, barely managing to stabilize the beam. Nana's blue eyes became the main light source, tracing a cool trail on the rock wall.
"This place is starting to look more and more like someone's abandoned basement," he muttered. "All that's missing is a few piles of rotten cardboard boxes and a 'No Dumping' sign."
Suddenly, he stopped.
"Listen."
Nana also calmed down.
At first there was nothing, only the sound of dripping water in the distance. But a few seconds later, a deep rumbling came from deep within the rock strata, like wind blowing through a tunnel entrance, or like some huge machine slowly turning.
"The sound of water?" he asked.
"Frequency analysis in progress." She closed her eyes for a second, and a blue light flashed. "The sound source is 30 degrees to the left, about 170 meters away. Preliminary judgment is that it is a flowing liquid with a flow rate of no less than half a meter per second."
"There really is a river?" His eyes lit up. "There are actually hidden currents under this desolate planet." Chen Hao took two steps, then suddenly stopped.
"Wait a minute." He turned and stared at the spare seedling leaning against the wall, his brow furrowed. "That leaf—did it twitch again?"
Nana's hand was still suspended in mid-air, less than ten centimeters from the rock face. She slowly withdrew her arm, a blue light flickering in her eyes for a moment before she pulled up the vibration records from the past ten minutes. The data bar stretched out a faint ripple in her vision, like spider silk ruffled by the wind.
“Low-frequency oscillations were detected,” she said. “At a frequency of 0.3 Hz, with periodic fluctuations, the possibility of geological subsidence is ruled out. The closest model is… underground fluid movement.”
"Water?" Chen Hao squatted down and poked the ground with his finger. "You think a river can spring up from this crack in the rocks?"
“We cannot rule out the existence of a hidden underground flow system.” Nana looked into the depths of the cave. “If there is long-term water seepage, the discoloration of the moss on the walls and the abnormal shaking of the plants can all be explained.”
Chen Hao plopped down on the ground and looked up at her: "So now we have two choices—either wait for the wall to collapse and be buried alive, or we take the initiative to go in and take a look?"
"The priority recommendation is to evacuate to a safe zone and continue monitoring."
"Come on," he waved his hand. "You make it sound like I don't want to go. I've tried lying down eight hundred times already, but when have I ever actually succeeded? Since we're going to be investigated sooner or later, I might as well go now while my legs are still strong enough."
He propped himself up on his knees, dusted off his pants, and said, "Besides, the little water we have at our base isn't enough to wash our faces in a day. If there really were a river, even if we couldn't drink it, we could save half the energy consumption for purification."
Nana didn't move.
He knew what she was waiting for.
"Fine, fine," he sighed, "Can I sign this? 'Voluntarily entering dangerous areas, bearing all consequences, and not holding the robot legally responsible'—are you satisfied now?"
The blue light flickered, as if it was trying to suppress a laugh.
“Let’s go,” she said.
The two walked along the main passage, the stalactites overhead becoming increasingly dense, the stone pillars hanging from the crevices resembling inverted spears. The ground began to slope, making their feet slippery, and Chen Hao had to grip the rock wall to inch forward. A loose pebble was sent flying by his foot, rolling into a side cave, only to be heard with a muffled thud several seconds later.
"This place is even harder to navigate than my apartment building's stairwell," he panted. "Who used to live here? The Bat Neighborhood Committee?"
Nana walked ahead, arms outstretched for balance. A slight humming sound emanated from her joints, particularly noticeable in the humid air. "The slope ahead increases to thirty-seven degrees; we recommend using the three-point support method for climbing."
"Speak human language."
"Use both hands and feet."
“I told you so.” He lay down and crawled up on all fours, his stomach churning painfully against the protruding rocks. “If I die here, my tombstone will say, ‘Here once stood an obese young man who tried to imitate a gecko.’”
Nana glanced back at him, said nothing, but simply reached out and tugged at his arm.
After traversing the steep slope, the passage suddenly narrowed, allowing only one person to pass sideways. Chen Hao was stuck in the middle, his ribs aching from the stones on either side, and he had to twist his body every few steps. "I feel like a dumpling stuffed into a straw," he muttered. "If I gain another five pounds, I'll probably choke to death."
“According to our calculations, your current size allows for a minimum width of 48 centimeters,” Nana said from behind him. “There is currently 61 centimeters of space remaining, which is ample safety margin.”
"You actually calculated that?"
"All obstacles have been modeled."
"So you have a 3D nude model of me in your head?"
"For use only in access assessment."
"Hey, you're quite serious." He squeezed through and slumped down as soon as he landed. "I need to rest for a bit, otherwise I'm afraid the next section is a vertical shaft, and I won't even have the strength to jump down."
Less than five minutes after resting, a strange noise came from afar.
At first, it sounded like the whistling of wind through a crack, so faint it was almost mistaken for tinnitus. Chen Hao raised his hand to signal for quiet, his ears perking up.
The sound came again.
It's not the wind.
It is water.
Slow, low, with a certain rhythmic echo, like a breath coming from the depths of the earth.
"Did you hear that?" he asked in a low voice.
Nana nodded, her blue light switching to scanning mode. She took a few steps forward, pressed her ear close to the rock wall, and spoke after a few seconds: "The sound source is 12 degrees to the left, about 170 meters away. The Doppler effect indicates that the flowing medium is continuous—it is most likely an underground river."
"A river?" Chen Hao grinned. "There really is a waterworks hidden under this desolate planet?"
"The scale and destination have not yet been confirmed."
"Then what are we waiting for?" He stood up, patted his butt, and said, "Let's go see if we can catch a fish for dinner."
There are no fish ecological records in this area.
"I was just using an analogy!"
"Understood. The metaphor function has been updated to the daily conversation library."
"Could you please not take this so seriously every time?"
"cannot."
Chen Hao rolled his eyes, but had already taken a step forward.
The further you go, the more humid the air becomes, and even breathing feels cool. The passage gradually widens, and the stalactites overhead decrease, replaced by large sections of peeling rock revealing dark brown surfaces. The ground is no longer flat, covered with pebbles, and in some places, it has even collapsed into shallow pits.
Chen Hao picked up a fist-sized rock and threw it forward.
With a "thud," it landed about five or six meters ahead. The ground was stable.
He threw another piece and took two more steps forward.
The third time it was thrown, the stone rolled a few times and suddenly disappeared into the shadows.
"Wait." He crouched down and reached out to touch the edge—not a pit, but a crack. A deep, horizontal trench lay before him, wide enough to swallow an adult.
He took out his small flashlight and shone it down. The beam swept across the water's surface, reflecting a ghostly green light.
"Holy crap... there really is water."
Nana approached the edge and began a full-range scan. Blue light swirled rapidly in her eyes, and data streams swept past silently.
"Confirmed to be an underground river section," she reported. "It is about five meters wide, with a flow rate of 0.7 meters per second, flowing from west to east. The depth exceeds the detection limit, and it is preliminarily judged to be a stable river channel, not temporary water accumulation."
Chen Hao lay down on the side and reached out to feel the breeze blowing up. "It's chilly... If I could jump in here in the summer, it would be so refreshing."
"The water temperature is 13 degrees Celsius, the oxygen content is low, and the concentration of suspended minerals is high, so it is not suitable for direct drinking."
"I was just saying," he said, withdrawing his hand. "But...where does this river lead?"
"The downstream path cannot be predicted. The topographic map breaks off here."
"So, everything ahead is unknown?"
"yes."
Chen Hao grinned: "Interesting."
He stood up and looked around. This space was much larger than the planting area, with towering rock walls whose tops disappeared into the darkness. The sound of flowing water was clearer here, like some kind of whisper, continuous and persistent.
“The water collectors at our base collect less water in a day than the flow from this river in a minute,” he said. “If we could divert some of the water up here, it would solve all our needs for irrigation, washing, bathing…”
"The difficulty of the diversion project and the structural stability need to be considered."
“I know it’s difficult.” He kicked at the pebbles at his feet. “But how will you know it can’t be done if you don’t try?”
He bent down, picked up a flat stone, weighed it in his hand, and tossed it onto the river.
The stone struck the water, bounced, and sank.
“Failed to skip stones.” He shrugged. “Looks like this river is pretty deep.”
Nana suddenly reached out and pulled him back.
The rock beneath their feet cracked open with a "crack," and pebbles fell into the water with a rustling sound.
“The edge is not strong enough to support the weight,” she said. “The area you just stepped on is below a cavity erosion zone.”
Chen Hao swallowed hard: "Thanks, I almost became the first human to fall into an alien sewer."
"The probability of death is estimated at 62 percent."
"You really think you can calculate that?"
"Routine risk assessment."
He chuckled twice, retreated to a safe area, plopped down, and casually tossed the stone fragments he had just skipped into a recycling bag.
“Fine, if we don’t want to get close, we won’t.” He looked up at her. “Then do you have a way to measure how fast the water is flowing? We can’t just assume we can use it because it’s flowing slowly.”
Nana paused for two seconds, then pulled a piece of metal from the back panel—it was the buoy shell left over from disassembling the drone. She quickly assembled it into a simple floating device, adding a counterweight at the bottom to ensure it wouldn't capsize.
"Prepare to deploy the test unit."
"Your habit of carrying spare parts around is starting to resemble that of a car mechanic," Chen Hao said, watching her work. "You could open a shop if you were just missing a wrench."
“Necessary redundancy.” She walked to the edge and carefully placed the buoy into the water.
The water flow immediately carried it forward. Nana activated the tracking program, and a blue light flashed continuously, recording the trajectory and speed changes in real time.
Fifteen seconds later, the signal disappeared.
“We’ve lost contact,” she said. “We suspect it has entered a bend in the river or encountered an obstacle. But data shows a stable flow, no turbulence, and a relatively straight channel.”
"So, you're saying the river is flowing quite steadily?"
"yes."
Chen Hao gazed at the dark water, his eyes gradually brightening.
"Where does this river lead..." he murmured. "Our base currently relies on drip irrigation for water. If only we could bring a section of it up here..."
Before he could finish speaking, he had already entered a state of planning, tracing lines in the air with his fingers and muttering "slope," "drop," and "pipe material."
Nana stood aside without interrupting.
After a long while, he suddenly turned to look at her.
"Do you think we could try building a raft?"
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