The soft thud from the cave entrance still echoed in his ears. Chen Hao didn't move, but simply pressed his body another half inch closer to the rock wall. He still held the flashlight in his hand, the switch unpressed, his knuckles a little sore.

Nana had already retrieved the drone and turned off all the lights. She stood three steps away from him, her head slightly tilted, as if listening to something.

"Is there any sound?" Chen Hao asked softly.

“That was a rock rolling down,” she said, “but there’s no further tremor now.”

"Sensors..."

“I was touched,” she interrupted. “Physical contact, not a false alarm.”

The two remained silent for a few seconds. The wind stopped, the snow stopped falling, and the entire valley was as quiet as if it had been frozen.

Chen Hao swallowed hard. "So...it went in?"

“Or maybe it’s out,” Nana said, “but we can only investigate further ahead.”

"Couldn't we wait until dawn to go in?"

"We can't wait. The clues will disappear."

Chen Hao sighed and looked down at his shoes. A thin layer of ice had formed on the surface, making a crunching sound as he stepped on them. He rubbed his foot against the rock, but the sound didn't subside.

"With my size, even taking a few steps feels like banging a gong."

“You do walk loudly,” Nana nodded. “That’s why I walk in front.”

She turned on her headlamp, adjusting it to the lowest brightness, and a beam of dim yellow light shone on the ice at the cave entrance. The snow piled up at the edge of the crevice was blown askew by the wind, revealing the bottomless darkness below.

Chen Hao followed behind, using the telescopic pole for support as he moved forward step by step. The ground was sloping, and his feet slipped; he almost knelt down, but luckily he managed to grab onto the crystal wall beside him. The thing didn't feel cold to the touch; rather, it was somewhat warm.

"Is this stone hot?" he asked.

“Local heat conduction anomaly.” Nana glanced back. “It might be an underground heat flow channel.”

They continued walking. Ten meters later, they reached the spot where the probe had stopped. A fork in the road appeared before them, and a faint breeze blew out from the left passage, carrying an indescribable smell.

"It smells like an old battery that's been stored for a long time." Chen Hao frowned.

“Metal oxidation.” Nana collected an air sample. “The internal space has been sealed for a long time, and the oxygen circulation is stable.”

"So, no one has touched it inside?"

"It's also possible that someone did it, but didn't leave any metabolic traces."

"You really know how to comfort people."

Nana didn't reply, turning to the left passage. She walked steadily, her footsteps almost silent. Chen Hao followed behind, dragging the pole, each step cautious, afraid of disturbing anything.

After walking another fifteen meters, the passage suddenly widened. There were more crystals on the rock walls, reflecting a faint light and casting shadows all around. Directly in front, a stone gate-like structure stood there, its surface covered with deep grooves.

"Is this man-made?" Chen Hao leaned closer to take a look.

“It wasn’t formed naturally.” Nana scanned the wall with the scanning module. “The scratches are of uniform depth and arranged in a regular pattern.”

The wall is covered with geometric patterns, some resembling spirals, others grids, and still others stylized numbers. In the center is a ring of circular symbols surrounding a triangle.

"This thing looks like a Wi-Fi password sticker." Chen Hao took two steps back. "Who would carve a password in a place like this?"

“It’s not a password.” Nana brought up the database interface. “These symbols have structural similarities to ancient Sumerian script, but the logic is closer to mathematical sequences.”

"You mean, this is some kind of encoding?"

"The probability is over 82%."

"Can you understand it?"

"Not at the moment. More samples are needed for comparison."

Chen Hao went around to the other side of the wall and found a raised platform in the corner with a flat stone slab on it. He shone his flashlight on it and saw several recessed button-like structures on the stone slab.

"Could this be a power switch?"

"Don't touch it," Nana said immediately. "Unknown system, triggering mechanism unknown."

"I'm just taking a look." He withdrew his hand. "How long do you think this place has been untouched? Hundreds of years? Thousands of years?"

"Based on the degree of rock weathering, it is estimated to be at least two thousand years old."

"Are we the first living creature to come in?"

“You are the second human to breathe,” she said. “I am a robot.”

"You really want to compete with me for this ranking?"

Nana ignored him and walked to the end of the passage. There, a circular device was half-buried in the ground, about two meters in diameter, with fine etched patterns on its surface, looking like a circuit board, but without a power interface or moving parts.

“The material is a high-purity silicon alloy,” she said after scanning. “It does not belong to any known common mineral assemblage.”

"Alien technology?" Chen Hao squatted down to take a look.

"The possibility of independent development of local civilizations cannot be ruled out."

"Local? This godforsaken place doesn't even have a single tree, how could they possibly build something like this?"

“A harsh environment does not mean that wisdom cannot be born,” she said. “On the contrary, it may prompt them to develop technology faster.”

Chen Hao stood up and dusted off his trousers. "So you mean, a bunch of ancient people dressed in animal skins, researching semiconductors while eating snow?"

"The path of civilization development is unpredictable."

"But how will they keep warm? What will they eat? They can't rely on faith to generate electricity, can they?"

“The murals may offer clues to these questions.”

"You actually believe that the graffiti on that wall can tell a story?"

Nana didn't respond; instead, she initiated a full-band scan. Her robotic arm moved slowly, recording data for each scratch line by line from top to bottom.

Chen Hao leaned against the wall, feeling a little tired. He took out his thermos and drank a mouthful of hot water, which scalded him, making him exhale sharply.

"You mean those footprints outside...could they be the descendants of these people?"

“Three hundred kilograms, standing upright on two legs,” Nana said, “does not conform to the human evolutionary model.”

"What is that? A mutation? A failed experiment?"

"There is currently insufficient data."

"Can't you just say something definite?"

"What I can confirm is that the device has no energy reaction, the mural has no dynamic changes, and we are currently safe."

"Thank you, I feel much more at ease now."

He put down the kettle and suddenly noticed a thin crack in the corner of the wall, which looked like it could be pulled. He reached out to try it, and as soon as his fingertips touched it, the entire wall made a low groan.

"Don't move!" Nana suddenly turned around.

But it was too late. The wall began to shake, and a faint blue light emanated from the cracks in the engravings, spreading out in concentric circles, as if some program had been activated.

"What did you touch?" she asked, walking quickly towards him.

"I just touched the crack! Who knew it was a touchscreen!"

The blue light lasted for about ten seconds, then went out. Everything returned to normal, as if nothing had happened.

"System response interrupted." Nana checked the data. "The instruction was not completed, possibly due to energy depletion."

"So it didn't start successfully?"

"Or it was activated, but we didn't notice."

Chen Hao stared at the wall, his heart pounding. "Do you think it was... recognizing someone just now?"

“It’s more like verifying the input pattern,” she said. “The position you triggered corresponds to the seventh column of the third section of the mural, which is the starting point of the sequence.”

"So I became the power button?"

"It was an unexpected turn of events."

"Next time, could you let me run a little further away before trying?"

Nana didn't answer, but walked towards the circular device. She noticed a new ring of marks on the bottom, like the imprint left after the flash of blue light.

“It’s changed here,” she said.

Chen Hao leaned closer to take a look. On the originally smooth base, a set of newly engraved symbols appeared, arranged in a similar way to those on the wall, but simpler.

"This... just came out?"

“Physical etching,” she confirmed. “Not a projection or illusion.”

"So, this thing is actually alive?"

“It has no vital signs,” she said, “but it responds to external stimuli.”

"Then you'd better write it down quickly, just in case it shuts itself off again later."

Nana immediately began recording. Chen Hao, on the other hand, looked around for any other changes. He discovered several newly formed small holes on the other side of the rock wall, arranged in a triangle.

He poked one of them with the telescopic pole, and the pole went in halfway before getting stuck.

"Hey, this hole swallowed my tools!"

Nana came over and took a look. "This isn't a decorative hole. There's a conductive structure inside."

"Then what about my pole?"

"I suggest giving up."

"This is the only guy I can use to scout ahead!"

"You can use your hands."

"My arms aren't that long."

"Then don't extend it that far."

Chen Hao rolled his eyes and pulled hard a few times, but the pole didn't budge. In the end, he had no choice but to let go and take a step back.

Just then, the ground trembled slightly.

The two stopped moving at the same time.

"An earthquake?" he asked.

“Energy fluctuation,” Nana said, looking at the reading. “It’s coming from below the device.”

The patterns on the surface of the ring lit up again, this time in a dark red, flashing on and off like a heartbeat.

"What is it going to do?" Chen Hao took a half step back.

“I don’t know.” Nana stood still. “But its reaction frequency is the same as the time interval between when you touched the wall just now.”

"You mean...it's imitating me?"

"More accurately, it is learning."

"After completing the next step, will there be an exam?"

The red light continued to flicker, the rhythm becoming faster and faster. Chen Hao felt a vibration coming from the soles of his feet, as if something was moving beneath him.

He looked at Nana. "Shouldn't we consider leaving?"

"Leaving now will result in the loss of critical data."

"But what if it explodes?"

"Then run."

"Can't you give me some reliable advice?"

“I’ve always been reliable,” she said. “It’s just that you don’t like to hear it.”

The red light suddenly stopped.

The entire cave was plunged into darkness.

The headlamp was still working, but the ring around it had completely gone out, leaving no trace of warmth.

A few seconds later, Nana's scanner received a signal.

"It's sending out data," she said. "In encrypted format, it's being parsed."

"Can you understand it?"

“Partial symbol matching successful.” She stared at the screen. “The content begins with—'Observer has arrived.'”