Nana's finger landed on the replay button again.

She pulled up the last signal segment and slowed it down three times. The waveform rose smoothly, then suddenly spiked by half a division before ending at 0.6 seconds, lasting less than a tenth of a second, before returning to zero. It wasn't attenuation, nor interference; it was a cutoff.

Chen Hao stood behind her, staring at the screen for a long time. He rubbed his eyes and leaned closer.

“It doesn’t look broken,” he said. “It looks more like…the lights are off.”

Nana nodded. "The last transmission used maximum power output, probably to ensure the information was delivered completely. It was immediately interrupted afterward, without attempting to reconnect or reduce the transmission frequency."

"This means it knows we can receive it."

"This also indicates that it has decided to discontinue the project."

The room quieted down. The sounds of the machines became exceptionally clear: the fans spinning, the hard drives reading and writing, and the faint vibrations of water flowing through valves in the distant pipes.

Chen Hao turned and walked to the tool cabinet, opened the bottom drawer, rummaged through it, and took out an old earphone, plugging it into the main control panel interface. He put one earbud in his ear, clipped the other behind his neck, and listened for a while with his head tilted to the side.

“It’s rhythmic,” he said. “It happens every thirty-seven seconds, repeating nine times. It’s not gibberish, nor is it natural electromagnetic disturbance. Who keeps track of their geological activity by the second?”

“I have already done a spectrum comparison,” Nana said. “We have ruled out the possibility of known celestial radiation, atmospheric ionospheric reflection, and underground mineral vein resonance. The signal frequency is in the civilian communication band, but the encryption method does not belong to any public protocol.”

"A channel for unregistered individuals?"

"Possibly."

Chen Hao took off his headphones and tossed them casually onto the table. "You said it was blocked by an obstacle before, does that mean the other side was hiding?"

“The signal path exhibits refraction characteristics.” Nana pulled up a 3D projection map. “The propagation direction has shifted by four degrees and seventeen minutes. Based on the terrain data, the source of the signal may be located below the ridge to the north or buried deep in the ice.”

"That thing can burrow into the ground by itself?"

"No. That means someone put it in there."

Chen Hao sat back in his chair, stretching his legs under the control panel, nearly kicking over a water bottle. He pulled his foot back, not bothering to steady it.

“So our current situation is—” he counted on his fingers, “first, there’s something we don’t recognize sending signals; second, it’s deliberately hiding and not allowing direct connection; third, after sending a few rounds, it suddenly stopped, as if someone had covered its mouth.”

"Logical chain is established".

“This isn’t a malfunction.” Chen Hao looked up at her. “Someone doesn’t want us to hear this.”

Nana's optical module subtly adjusted its angle, as if observing him. "It's currently impossible to confirm whether there's hostility. However, the possibility of surveillance or manipulation cannot be ruled out."

"I don't believe in free lunches." Chen Hao picked up a pen and drew a circle on a piece of paper on the table. "If a kind person had left a beacon, it would have gone off decades ago. But it just so happens to start knocking on the wall as soon as we're fixing the equipment. What a coincidence."

He paused, then added, "Everything we're doing right now is starting from scratch, groping in the dark. What if they've been watching us all along? Didn't I just tell you to start a deep investigation, and you didn't agree?"

“Active detection would reveal our location,” she said. “Once a tracking wave is launched, the other side can use triangulation to pinpoint the base’s coordinates.”

"Are you still keeping the listening program running?"

"Remaining in passive receiving mode. No enhanced scanning performed."

Chen Hao nodded, his expression not very good. "In other words, we're like two people crouching by a window peeping in, only to discover that there's another pair of eyes watching from outside the curtains."

"The metaphor is reasonable."

He stood up and paced back and forth in the room twice. His footsteps echoed hollowly on the metal floor. He stopped at the door and glanced back at the main screen.

The signal frequency remained empty. That straight line had persisted for twenty-three minutes.

“I think we can’t touch data acquisition anymore,” he said. “When we were measuring the aluminum plate just now, we used a vibration motor and a pressure tank. These devices generate electromagnetic leakage when they are running. If someone over there was really listening in, we would have been shouting ‘We’re here’ through a megaphone just now.”

"You mean to suspend all experiments?"

"Let's at least find out who's outside before we talk about it."

“The database update is not yet complete,” Nana said. “The low-temperature toughness parameters still need to be supplemented and verified.”

“Then let’s make do with the existing data for now.” Chen Hao walked to the communication panel, his hand hovering over the call button. “The tools can be repaired slowly, and the ship can be delayed for a few more days. But if we lure the enemy in, we’ll all be doomed.”

He paused for a moment and withdrew his hand.

“We can’t call them directly,” he said. “If the call is intercepted, it will implicate them.”

"I suggest organizing the existing information into a briefing before convening a meeting," Nana said. "This will help avoid causing unnecessary panic."

“Yes.” Chen Hao nodded. “Let’s clarify the evidence first. For example, can we pinpoint the exact location of this refraction path?”

Nana brought up the terrain model and matched the signal incident angle with the surface undulations. The screen rotated a few times, and the cursor locked onto a depression near 41 degrees north latitude.

“There is an underground cavity structure here,” she said. “It is about 15 meters in diameter and covered with an eight-meter-thick layer of permafrost. The entrance has been buried, but thermal imaging shows that there is still a slight residual heat inside.”

"People live there?"

"Insufficient to support the long-term survival of living organisms. But if it is just an automated device, it can keep operating for several months."

How far is that place from us?

"The straight-line distance is 6.7 kilometers. Walking requires crossing two cliff sections, which are inaccessible by regular vehicles."

Chen Hao whistled. "They've got a good eye for spotting things. They can't get in, they can't hit them, and they can still see what's going on here."

"It cannot be ruled out that remote monitoring cameras have been installed."

"I was wondering why there was so much dust on the ceiling of the workshop lately."

Nana turned to him. "Are you implying someone climbed over the roof?"

“I don’t know,” Chen Hao shrugged. “But when I was fixing the ventilation duct yesterday, I found scratches on the outer filter, like it had been rubbed by a hard object. At the time, I thought it was from the wind, but now that I think about it…”

"I will retrieve last night's infrared recordings."

"Don't check now." Chen Hao pressed her hand down. "What if it triggers an alarm? Check it later."

He walked back to the control panel, picked up the piece of paper he had circled earlier, tore it in half, then tore it into four pieces, and finally crumpled it into a ball and threw it into the trash can.

“We now know three things,” he said. “First, the signal came from a man-made device; second, it was deliberately concealed; and third, it stopped sending after we started testing. These three points can only lead to one conclusion.”

Nana looked at him.

“Someone is watching our reaction,” Chen Hao said. “Whatever we do, it changes accordingly. This isn’t a cry for help; it’s a test.”

The room temperature seems to have dropped a few degrees. The sound from the air conditioner vents has decreased; it may have automatically switched to energy-saving mode.

Nana switched the screen to the background monitoring interface, set the refresh rate to the lowest setting, and only kept basic log records. She shut down all external interfaces, including the backup antenna and emergency beacon.

“We have entered silent mode,” she said. “All non-essential equipment except for basic life support systems has been shut down.”

“Very good.” Chen Hao leaned against the wall. “Next, we need to put on a show.”

What play?

"Pretend we haven't found anything. Continue repairing the ship, replenishing supplies, and working as usual. Let them think we're still working blindly."

What if the other party sends another message?

"Ignore them. Let them lose their temper."

"The risk is that we might miss a real distress signal."

"Let's take a gamble then." Chen Hao smiled. "Anyway, I've never passed an exam before, so I'm used to it."

He walked to the door and suddenly stopped.

"By the way, are you having bean flour porridge for breakfast again tomorrow?"

"According to the nutrition facts, tomorrow's breakfast should be oatmeal mixed with protein powder."

"Oh." Chen Hao opened the door. "Then remember to use less salt. Last time it was so salty that I had to get up in the middle of the night to fill the container with water, and I almost caught Karl secretly carrying welding rods to the warehouse."

The door closed.

Nana remained seated, without turning around. Her fingers lightly glided across the keyboard, bringing up an unmarked log file. It was a sensor recording from 3:42 AM, containing a frame of abnormal data that had not been cleared.

In the image, an unusual red dot appeared in the thermal image of the northern ridge. It was very small and lasted for less than a second. But it moved.

It's not a device malfunction.

It wasn't just a rustling of leaves.

Nana saved this data separately, naming it "Temporary Cache_07". She neither deleted it nor reported it.

In the bottom right corner of the screen, the time jumped to 4:01.

Her optical module flashed.