The submersible slowly advanced through the black water, the reflection from the rock formations 500 meters ahead growing stronger. On the screen, the nearly complete circular metal structure, like an old iron disc buried for who knows how many years, sat quietly against the side of the cliff.

“The magnetic field is still fluctuating,” Nana said, staring at the data stream. “But the frequency is stable, still 3.7.”

Chen Hao grinned: "It didn't turn hostile, which means our greeting just now worked."

Carl gripped the joystick: "But the water flow is wrong. The pressure on the left side suddenly increased, like something is pushing us."

“It’s not an ocean current.” Susan looked at the sonar chart. “It’s a local circulation, circling around that disc, like… a fan blowing in front of a security door.”

"It's quite energy-efficient," Chen Hao said. "No need for guards; it can just be swept away by water."

Nana pulled up a fluid dynamics model: "Ancient large-scale facilities often used hydrodynamic barriers to prevent accidental entry. To get close, one can only crawl along the bottom and avoid the main vortex."

“Then let’s use the wall as a step.” Chen Hao patted the control panel. “Karl, take your time. Don’t let it think we’re deliverymen coming to collect a signature.”

Carl didn't speak, but reduced the thruster power to its lowest setting, and the submersible, like a lame crab, slowly moved closer to the rock wall. The new coating on the outer shell shimmered under the low pressure, like moss covered in night dew.

When we got within 200 meters, the image became clear.

It was a circular gate array about ten meters in diameter, its surface covered with thick sediment. Six symmetrical grooves were visible along its edges, and in the center was a recessed disc area covered with symbols identical to those on the map fragments. The lines were winding and meandering, yet their arrangement conveyed an inexplicable sense of regularity.

"Don't touch it," Susan said immediately. "When the robotic arm grazed past, the edge flashed blue."

“An electric arc reaction,” Nana added. “An energy threshold triggering mechanism; any non-standard contact will activate the defense system.”

"Does that mean I'll get beaten up for touching you randomly?" Chen Hao asked.

“It could trigger a localized landslide,” Nana said, “or activate anti-intrusion procedures.”

"What should we do then? Knock on the door and shout 'Is anyone home?'"

“We can use low-frequency sound waves to scan,” Susan suggested. “We can read structural information without making contact.”

Three minutes later, the acoustic image was transmitted back.

Nana quickly compared the data with the database: "Match successful. This is the standard design of the 'Unsealing Gate Array,' commonly found at the entrances of advanced civilizations in the late pre-Geological Period. It requires the activation of six nodes in a specific order to open."

"How many nodes?"

Six.

"Are there any prompts? For example, which one to press first?"

"No. If more than three incorrect attempts are made, the system will either permanently lock or release the high-voltage pulse."

The cabin was silent for a second.

"This is just an oversized combination lock," Chen Hao said, scratching his head. "It's just missing a note saying 'initial password'."

“The problem is what the password is,” Carl said. “We can’t just try it randomly.”

Chen Hao leaned back in his chair, his eyes fixed on the array of symbols on the screen. The lines radiated outwards in concentric circles, somewhat resembling the rhythm of the signals they had previously received.

He suddenly sat up straight: "Wait... 3.7."

"What?"

“We previously sent a signal, flashed once, paused for 3.7 seconds, then flashed again. It responded with 3.7 eight times. Is the symbol on this door also related to this?”

Nana immediately brought up the image analysis program: "Convert the symbol spacing to a time unit, and perform periodic detection based on 3.7 seconds..."

The progress bar has finished.

[Discovery of Periodic Sequences]

The intervals between the six groups of symbols are 3.7, 7.4, 11.1, 14.8, 18.5, and 22.2 seconds, respectively.

[This forms an arithmetic sequence with a common difference of 3.7]

“It really is arranged chronologically,” Susan said softly.

“So this isn’t a pattern, it’s a clock,” Chen Hao laughed. “The kind with an alarm clock.”

“The nodes must be activated in this order,” Nana confirmed. “The interval between each step must be strictly controlled at 3.7 seconds, otherwise the rhythm will be disrupted and an alarm may be triggered.”

Who operates it?

“I’ll do it.” Chen Hao reached out and took the control lever from the robotic arm. “None of you have a steady hand like me—especially when you’re hungry.”

No one objected.

The first node was located in the lower left corner, half-buried in the mud and sand. Chen Hao carefully extended the robotic arm over and pressed the timer the moment it touched the node.

drop.

The first groove lit up with a dark red light.

wait.

One second. Two seconds. Three seconds.

Three seconds and seven seconds.

The robotic arm moved again, pointing to the second node.

Click.

Blue light flowed.

Keep waiting.

The third, fourth, and fifth points are lit up in sequence. With each step completed, the water around the gate becomes quieter, as if the entire structure is adapting to this rhythm.

We've reached point six.

The last node is directly above, positioned too high, and can only be barely reached by the robotic arm extending to its limit.

“The angle is a bit off,” Carl pointed out. “Just five centimeters higher and it will be just right.”

“I know.” Chen Hao held his breath. “Not even a millimeter off.”

He slowly adjusted the boom, his fingertips hovering over the control lever.

Three seconds and seven seconds passed.

He pushed the control stick.

The robotic arm lightly touches the node.

Buzz—

The entire ruins suddenly trembled.

All the lights simultaneously turned a ghostly green, contracting inwards along the annular groove. The central disc slowly cracked open, a vertical slit splitting from top to bottom, like eyes opening.

Dim light seeped out from inside and shone on the front window of the submersible.

“It’s open,” Susan said softly.

"It's good that it didn't explode." Chen Hao let go of her hand and then realized that his back was soaked.

“The internal pressure is balancing.” Nana monitored the data. “The seawater outside the door is isolated by some kind of force field, and a stable vacuum zone has been formed in the passage.”

"Can we go in now?"

"Yes. But it is recommended to keep the speed low to avoid disturbing the residual energy field."

Carl slowly pushed the throttle, and the submersible glided forward, passing through the newly opened gap.

After entering about fifteen meters, the external camera showed that the gate behind them was still slowly closing, but not completely shut, leaving a gap large enough for passage.

“It didn’t lock us up inside,” Chen Hao said. “It was quite reasonable.”

“Maybe it’s just the program setting,” Nana said, “allowing one entry and exit window.”

"So that means if you stay here too long, you'll really get locked out?"

"The possibility exists."

"Then let's hurry up." Chen Hao turned to look at the three of them. "Don't look back, just keep moving forward."

The walls inside the passage were smooth, their material indistinguishable, and they reflected no light, as if all color had been absorbed. At intervals along the ceiling, there were faint light sources, just enough to make out the path ahead.

"How long has it been since these lights were turned on?" Susan asked.

“At least tens of thousands of years,” Nana said, “but the power supply system is still running.”

Who charged it?

"Unclear. Energy type unknown."

The submersible continued forward; the ground was flat and there were no obstacles. Until a fork in the road appeared ahead.

All four directions are exactly the same, without any markings.

"Which side should we choose?" Carl stopped the machine.

"Whatever," Chen Hao said. "They all say 'Dead End' anyway."

“Not necessarily.” Nana pulled up the sensor data. “There is a very slight change in airflow in the right channel, and the temperature is 0.3 degrees higher.”

"Where there's wind, there's an exit?"

"It may also lead to the main control room."

“Then let’s turn right,” Chen Hao decided. “What if we happen to stumble upon an ancient air-conditioned room? I’ve been wanting to take a hot shower for ages.”

Karl turned the rudder, and the submersible entered the right-hand passage.

After walking less than fifty meters, a symbol array suddenly lit up on the wall in front of us, exactly the same as the one at the entrance, but arranged differently.

"Again?" Chen Hao frowned.

“This time it’s dynamic,” Nana said. “The symbols are moving.”

Sure enough, the lines began to slowly rotate, like some kind of countdown.

"Is it going to be another mystery to solve?"

“It’s not a puzzle,” Nana quickly analyzed. “It’s an identification process. We’re being scanned.”

"Sweep what?"

"identity."

Do we have ID cards?

“There is no registration record,” Nana said. “The system is determining whether to allow passage.”

The symbols on the wall spun faster and faster, until a recessed handprint outline appeared in the center.

"It wants us to put our fingerprints on it?"

"It might be a biometric authentication interface."

"Shall I give it a try?"

"No!" Susan stopped him. "What if they drain all the blood?"

“Then let the robot do it.” Chen Hao looked at Nana. “Didn’t you say you could simulate human physiological characteristics?”

“We can try,” Nana said, untying the wires. “But I can only move my right hand.”

She slowly extended her robotic arm into the groove.

The symbol stopped rotating the moment it made contact.

The walls emitted a low rumble.

Click.

A crack appeared in the ground ahead, and a platform rose up with a small box on it.