Thump, thump, thump.

Three taps, then a pause.

Three times.

The knocking started again, closer than before. Chen Hao didn't move, his hand pressed against the wall, his knuckles slightly clenched. He glanced at the timer on his helmet; the numbers were ticking normally, but the sound felt like it was drilling straight into his brain, hammering at his nerves.

"Don't make a sound," he said. "Listen."

The other three immediately fell silent. Only the hissing of oxygen circulating and the intermittent tapping filled the passage. It wasn't an echo, nor was it the cracking of rock; it was simply something rhythmically striking a hard object.

“Three times again,” Carl said quietly. “Same as last time.”

"That means it knows the way," Chen Hao said. "It's either a government office or something alive."

"If it were alive, it wouldn't have survived for 80 million years without eating or drinking?" Susan frowned. "It should have died long ago."

“I think so too,” Chen Hao nodded. “So it’s most likely automatic. Nana, did you detect any vibration source?”

Nana's eyes flashed with a blue light, and after a few seconds she spoke: "The signal is scattered, and we cannot pinpoint the exact location. However, the sound wave frequency is close to the resonance range of the diving suit, so we suggest maintaining low-frequency communication mode."

"Then let's not use voice broadcasts." Chen Hao raised his hand and made a cutting gesture. "From now on, communicate with gestures unless it's an emergency."

They continued forward, hugging the right wall, slowing their pace. The lights were dimmed, illuminating only a few steps ahead. The flashes on the ground continued, the rhythm steady, as if acknowledging they had passed the previous test.

Less than ten meters into the straight passage, a buzzing sound suddenly came from my headphones.

Low frequency, slow, and synchronized with the heartbeat.

Chen Hao stopped abruptly. The sound lasted only three seconds before disappearing. But everything went black before his eyes, and the next second he saw himself buried under rubble, the ceiling above him collapsing, the oxygen reading plummeting, and the helmet alarm blaring.

He opened his mouth to shout, but no sound came out.

The hallucination came and went quickly. Three seconds later, his vision returned. He found himself still standing in the same spot, gripping the wall, his knuckles white.

"Stand against the wall!" he immediately ordered in a low voice. "Everyone close your eyes! Don't look at anyone else!"

Susan was leaning against the wall, breathing rapidly. Carl crouched down, his hands covering his head. Nana stood still, the system interface scrolling rapidly with data.

“Abnormal brainwaves,” she said calmly. “All four people simultaneously experienced intense fear, lasting approximately 2.8 seconds. This was determined to be psychological interference induced by external sound waves.”

"I knew it." Chen Hao took a breath. "This place doesn't allow people to walk properly."

He closed his eyes and began to recall the training content. He had originally intended to slack off during that psychology class in Chapter 689, but Nana singled him out as a demonstration, forcing him to participate fully. Now, it was proving useful.

"Feel the wall," he said. "Don't let go. Feel the temperature and texture of the stone."

The three of them slowly reached out and pressed their hands back against the rock face.

"Take a deep breath," Chen Hao continued. "Inhale—hold your breath—exhale. Three times."

Nana gently patted his shoulder, once, twice, three times, keeping time. The others adjusted their breathing to the rhythm.

"I'll speak, you repeat." Chen Hao opened his eyes. "I am Chen Hao, I am on a mission, I am safe."

"I am Susan, I am on a mission, and I am safe."

"I am Carl, I am on a mission, and I am safe."

"This is Nana. I am on a mission. The system is functioning normally."

The last sentence was added by Nana herself, and her tone was completely flat, which almost made Karl laugh.

"You can open your eyes now," Chen Hao said. "Don't move around. First, make sure you're somewhere."

The four slowly opened their eyes, their gaze refocusing. The lighting was still dim, the passageway unchanged, but the feeling of being watched by something was gone.

“I just saw a big fish swallow me whole,” Carl said. “That’s fucking outrageous.”

“I dreamt that the base exploded, and no one answered my cries for help,” Susan’s voice was a little hoarse. “Like the last time I went deep into the ocean.”

"I'm fine," Chen Hao said. "It's just that I feel like I'm going to suffocate, the same old story."

Nana retrieved the scan results: "There is a ring-shaped crystal array at the top in front, about 80 centimeters in diameter, which is releasing pulse waves at an extremely low frequency. The database matching results show that this is an emotional resonance device commonly used in ancient civilizations, specifically designed to stimulate fear memories in the human subconscious."

"So, it specifically places things you're most afraid of?" Carl swallowed hard.

“Yes,” Chen Hao said. “And it looks like it can learn. The first time it takes two seconds, the second time it might take five seconds. If it triggers again, it might just pin someone to the ground.”

"Then what do we do? Go around it?"

“We can’t go around it.” Chen Hao looked at the end of the passage. “There’s only one road on the right. The slope on the left is all mud; once you step in, you won’t be able to get back up.”

Nana proposed a solution: "We can try using the communication module to emit anti-phase white noise to interfere with its sound wave frequency. However, someone needs to be within three meters of the device to perform calibration."

"I'll go," Chen Hao said. "You guys wait for the signal."

“Too dangerous,” Susan objected. “What if you get pulled into the hallucination again?”

"That's why I need to train first." Chen Hao grinned. "Besides, am I not fat? I have a lot of fat, so I can take a beating."

No one responded. He flexed his wrists and took three steps forward.

The buzzing sounded again.

This time the sound was deeper and lasted longer. Chen Hao's vision went black instantly. This time it wasn't just ruins, but the entire base had collapsed, seawater was flooding in, and he swam desperately, but his limbs felt like they were bound, and he couldn't move. All he could hear were alarms and cries, and his own increasingly weak breathing.

But his left hand was still pressed against the wall.

The sense of realism is still there.

He gritted his teeth and forced himself to say the words: "I am Chen Hao, I am on a mission, I am safe."

Once, twice, three times.

The white noise in the helmet suddenly started, like someone stuffing cotton wool into his ears. The hallucinations began to distort, the sounds receded, and the images shattered.

He opened his eyes; his fingers were still on the wall, and his body hadn't fallen.

“Success,” he said into the communicator. “The jamming is effective. You guys hurry over, don’t linger.”

The three quickly crossed the high-risk zone. Nana brought up the rear, adjusting the signal strength as she retreated. The blue light from the crystal array flickered a few times before gradually dimming.

For the last stretch, Carl walked the fastest. But just as he was about to leave the area, his glove scraped against the wall, making a slight scraping sound.

In that instant, the residual fluctuations of the crystal swept across the tail of the line.

Susan stopped in her tracks, her face turned pale, and her body stiffened.

"Susan!" Chen Hao turned around, grabbed her wrist, and squeezed it hard. "Wake up! What did you eat this morning?"

She didn't react.

"Answer me!" he raised his voice, "What flavor of seaweed cake did you eat?"

“…Spicy.” She finally spoke, her voice trembling.

“Yes, spicy.” Chen Hao stared into her eyes. “You still thought it wasn’t spicy enough, so you added double the amount of chili powder, which made you cough. Remember that?”

She blinked, and her breathing gradually returned to normal.

Nana immediately switched modes, and a recording played in her helmet: "Last night's dinner at camp was stewed fish. Karl took your share, and you said you were going to tell the captain, but he went to the kitchen to apologize himself..."

These are snippets of everyday conversation, lighthearted and mundane.

Susan's shoulders relaxed.

"Okay," she said, "I'm back."

The four continued forward until they entered a relatively open stone corridor, at which point they stopped. The passage behind them was now quiet, and the crystals no longer glowed.

Chen Hao leaned against the wall and took a breath.

“This thing is even more disgusting than a machine gun,” he said. “It doesn’t hit people, it only hits their brains.”

“But it has weaknesses,” Nana said. “It relies on environmental resonance. As long as interference signals are prepared in advance, it can be cracked.”

“Next time, let it hear me sing a song off-key first,” Chen Hao said. “I guarantee I’ll shake it to pieces.”

No one was laughing. Everyone was too tired.

After a short rest, Chen Hao looked up and gazed ahead.

The stone corridor splits into two paths, one straight ahead and the other sloping downwards. At the entrance is a half-open metal door with rusty edges.

"Which way?" Carl asked.

Before Chen Hao could speak, the knocking sound started again.

This time it's not far away.

It was in the wall right above their heads.

Thump, thump, thump.

Three taps, then a pause.

Three times.

Chen Hao looked up and slowly reached for the tool bag at his waist.