Thump, thump, thump.

The sound from inside the wall came again, this time clearer. Chen Hao looked up at the gap above the metal door, slowly pulling his hand out of his tool bag, his knuckles brushing against the edge of his helmet.

He turned around, faced the three people, raised his right hand, and pushed forward with three fingers together – this way.

Susan frowned, glanced at the straight passageway ahead, then at the half-open rusty door. She opened her mouth as if to say something, but Chen Hao immediately put his index finger on his mask and shook his head.

Unable to speak.

That psychological interference was terrifying; nobody knows if it would drive someone insane if it rang again. The voice signal might have been the trigger.

Nana stood in front of the door, her palms pressed against the doorframe. A blue light flashed in her eyes, then quickly went out.

“The structure is stable,” she said. “There are air cavities inside, so it’s not a dead end.”

Carl tapped the door gently with a wrench, and rust flakes fell off. He took a deep breath and reached out to push it.

squeak--

The door was pushed open a short distance, revealing a ramp behind it. The steps descended, their surface covered with a layer of dark sediment that left shallow imprints when stepped on.

Chen Hao took the lead, stepping onto the ramp. He felt a slight slip under his foot and immediately crouched down, holding onto the wall for support. The metal wall felt somewhat soft, as if it had oxidized excessively after being soaked in water for a long time.

"Walk slowly." He turned back and gestured. "One by one, don't stray too far apart."

The four of them lined up and descended the slope. The lights were dimmed, illuminating only a few meters ahead. The sounds overhead ceased, but no one dared to relax.

After walking for about ten minutes, the slope suddenly widened. At the end was an arched gate, embedded in the rock wall, its surface covered with cracks. In the center was a raised stone tablet, engraved with twisted symbols.

Nana took a few steps closer and reached out to touch the edge of the stone tablet.

“This is the writing of an ‘end-stage human civilization’,” she said. “It’s recorded in the database. They liked to hide important information inside building structures.”

"Can you understand it?" Chen Hao asked.

“Partially yes.” Her eyes lit up with a faint blue light again. “It says here: ‘Knowledge belongs to those who come after; machines safeguard the truth.’”

“Sounds like an epitaph,” Carl said quietly.

"It could be advertising copy," Chen Hao grinned. "They've been trying to trick people into buying things in stores for 80 million years?"

No one laughed. The oxygen reserve showed 75% remaining; time was running out.

Nana continued reading: "Below is the Chamber of Memories, which may be entered by those who possess a resonance frequency."

"Resonance frequency?" Susan looked at Chen Hao. "Was it that knocking sound just now?"

Chen Hao thought for a moment, then took off his gloves and gently tapped the stone tablet with his fingertips.

Once, twice, three times.

Pause for two seconds.

Three more times.

The stone tablet trembled. A pale yellow light seeped from the cracks, spreading along the veins. The entire tablet began to glow, then emitted a muffled thud.

Click.

The arched gate splits open from the middle, slowly receding into the rock strata on both sides.

Inside was a circular hall. The walls were inlaid with many small crystals, emitting a soft white light. The floor was paved with metal plates, the joints fitting together perfectly. In the very center was a platform, upon which rested three sealed containers and a larger stone tablet.

Chen Hao stepped inside, his shoes making a crisp echo as they hit the metal plate.

“This place doesn’t look like a graveyard,” he said. “It’s more like… a database.”

Nana had already walked up to the containers. Each box was about thirty centimeters square, with intricate patterns engraved on its surface and a groove in the center.

“It requires physical contact and a specific rhythm to activate it,” she said. “You can’t pry it open forcefully, otherwise it will self-destruct internally.”

"Or should we knock?" Carl asked.

"Yes. The rhythm must be precise."

Chen Hao put his gloves back on and stood next to the first container. He closed his eyes, recalling the sound from earlier, and gave his wrist a slight twitch.

Thump, thump, thump.

Short, forceful, and evenly spaced.

A flash of blue light appeared on the surface of the container, and the lid popped open a crack. He reached out and lifted it, revealing a silver-gray device inside, shaped like a battery, but with flowing light patterns on its surface.

"Energy core?" Susan leaned closer to look.

“Prototype.” Nana picked it up and flipped through it. “Its output efficiency is sixteen times that of modern equipment, and its size is only one-third.”

"Can it be used?" Carl asked.

“Theoretically, it is possible,” she said, “but it’s still running, which means the system is not shut down.”

Chen Hao opened the second container. This time it contained transparent chips, stacked in a pile, each chip as thin as paper.

"What's this?"

“Information storage medium.” Nana held a piece between her fingers. “Similar to a hard drive, but using quantum encoding. I’ll try to read it.”

She closed her eyes and placed her palm on the chip. A few seconds later, her eyelids twitched.

“Found it.” She opened her eyes. “A complete archive of their civilization. Including their social system, their technological development path, and… the reasons for their demise.”

"Get to the point," Chen Hao asked.

“They didn’t go extinct due to war or natural disasters,” Nana said. “But because the consciousness uploading experiment went out of control. Most people chose to leave their physical bodies and enter the virtual world. The real cities gradually became deserted, and in the end, only the robots that maintained the system remained.”

"So who built this place?" Susan asked.

“Robots,” Nana pointed to the network of crystals on the wall, “They’re left behind to organize history, waiting for the next intelligent race to discover them.”

Chen Hao looked at the third container. It hadn't been opened yet.

He reached out and pressed the lid, preparing to knock again.

"Wait a minute." Susan suddenly grabbed his wrist. "Don't you think...it's going too smoothly?"

"What's wrong?" Chen Hao asked. "Nothing collapsed, nothing exploded, nothing smoked along the way, not even a ghost."

“It’s precisely because it’s too quiet,” she said. “Where are the mechanisms ahead? The alarms? The defense systems?”

Carl nodded: "Yes, such an important thing, is it just left here for people to take as they please?"

Nana glanced around: "It certainly doesn't make sense. But the scan results show no hidden traps. The energy source is concentrated in the devices themselves, not in the surrounding environment."

“Maybe they’re not afraid of being stolen,” Chen Hao said. “What they’re afraid of is that no one will come.”

He tapped the container again.

Three strokes, pause, three strokes.

The lid rose. Inside was a metal book with a star map engraved on the cover.

He took it out and opened it; the first page had several large characters written on it:

"We were wrong."

Below is a brief record:

"When everyone chooses immortality, the meaning of life disappears. We delete pain, but we also delete joy. We eliminate death, but we make existence worthless. If you see this, please remember—living is the answer itself."

Chen Hao closed the booklet and took a breath.

“This isn’t treasure,” he said. “This is a will.”

“It’s also a warning,” Nana said.

“But the technology is real.” Karl pointed to the energy core. “Once it’s brought back to base, it can rewrite the entire energy system.”

“Documents can also solve many mysteries,” Susan said. “For example, why are there such relics on the seabed, and why were they only discovered now?”

Chen Hao looked down at his backpack. He put the two energy cores and all the chips inside, and wrapped the metal booklet separately and stuffed it into a compartment.

“Take it back,” he said. “Study it first, then decide how to use it.”

Nana walked to the large stone monument and placed her palm on it. Blue light spread along her arm, lasting for five seconds before receding.

“The 3D modeling is complete,” she said. “The data has been backed up.”

"Let's go." Chen Hao patted the platform. "The mission is half complete. The rest is getting out alive."

The four returned the way they came. The slope was still slippery, but they walked faster than they had come. They had 70% oxygen left, enough to last until they reached the exit.

Just as Chen Hao reached the metal door, he suddenly stopped.

He turned and looked into the depths of the hall.

"What's wrong?" Susan asked.

“I think…” he narrowed his eyes, “that voice just now wasn’t a warning.”

What is it?

"They're saying hello."

Before the words were finished, a soft sound came from the hall behind them.

It's like a switch being pressed.

The crystals on the wall lit up simultaneously, changing color from white to blue.

In the center of the platform, the last unopened container opened by itself.