Chen Hao had just placed the thermos on the table, his shoulders still tingling. He was about to sit down and catch his breath when Nana's voice rang out.

"Abnormal temperature in the left shoulder area."

He paused for a moment, "Again?"

“The data is reproduced,” Nana said. “It completely overlaps with the location where the diving suit was hit last time, with a temperature rise of 0.3 degrees Celsius, lasting for 0.8 seconds.”

Susan leaned closer and brought up the screen. "It's not frictional heat? Nor is it a device leaking electricity?"

“Rule it out,” Nana replied. “The energy source is neither external nor internal to the system. It’s more like… some kind of pulse coming from the direction of the rock strata.”

Carl looked up at the surveillance image. "That area is a slope fissure zone; theoretically, there shouldn't be anything there."

“But now I have it.” Chen Hao scratched his head. “It touched me and got warm. Isn’t that a bit too forward?”

"The sonar is scanning." Nana switched the screen. "The multi-band superposition results show that there is a cavity structure behind the crack, about 80 meters in diameter, and the depth cannot be determined."

A blurry black shadow appears in the image, resembling a hollowed-out mountain.

“It doesn’t look like it was formed naturally.” Susan frowned. “The edges are too regular.”

"So," Chen Hao grinned, "that rock we bumped into earlier was actually knocking on the doorframe of some door?"

No one responded.

Half a minute later, Carl said in a low voice, "Your analogy makes me not want to go into the cave."

"I'm not using this as a metaphor," Chen Hao said, spreading his hands. "I'm just reminding everyone that if someone lives inside, it wouldn't be very polite for us to just barge in like this."

“We can send the detector first,” Susan suggested. “Let’s check within a 50-meter radius for any electromagnetic interference or biological activity.”

“Okay.” Chen Hao nodded, “but don’t expect it to come back alive.”

The small submersible was launched into the water within ten minutes.

For the first forty meters, everything was normal. The camera's feed showed only rocks and slowly flowing sediment. At forty-five meters, the signal started to flicker. The image distorted twice, then turned into static.

The last frame of the image shows a narrow opening hidden at the bottom of the slope, covered with dark vine-like material, with a slit in the middle, as if it had been pushed open halfway.

Then the signal dropped.

"Lost contact," Nana reported. "Last known location was seven meters from the predicted cave entrance."

"Eaten?" Carl asked.

“It’s more likely to be strong magnetic field interference,” Nana said, “similar to the environmental characteristics during the last X-7 ore activation.”

Chen Hao stood up, stretched, and his bones cracked.

"It seems I'll have to go there myself."

“You just finished testing the equipment, and you haven’t recovered your strength yet,” Susan stopped him. “You’ll have to wait at least six hours.”

“We can’t wait any longer.” Chen Hao pointed to the data recorded on the helmet. “That thing just touched me. It knows we’re watching it. If we wait any longer, the door might close.”

“You’re tired too,” Susan insisted. “It’s safer to let the robot scout the way.”

“Robots can’t judge unexpected situations.” Chen Hao shook his head. “For example, if a giant octopus suddenly jumps out, it can only take pictures and then get bitten to pieces. Humans are different. Humans can run, play dead, and cry out for help—these are all survival skills.”

Carl chuckled. "You're really confident."

“I’m not confident,” Chen Hao said, putting on his coat. “I’m just heavy and not easily washed away.”

Five minutes later, the four people were dressed and stood at the edge of the platform.

The new diving suits are now in full operation. The X-7 alloy shell has a matte silver-gray finish, and miniature propulsion nozzles have been added to the joints. The oxygen tanks have been reduced in size, but their endurance has doubled.

"Protection mode is activated," Nana prompted. "Microcurrent barrier is ready."

"Where's the navigation?"

"The AI ​​path memory function has been upgraded and can now mark complex terrain."

"Let's go then." Chen Hao took a deep breath, leaned back, and jumped into the sea.

The others followed closely behind.

The descent was quiet. The light disappeared quickly, leaving only a ring of white light from the helmet lamp. The seabed was clearly undulating, with more and more cracks, like wounds splitting open in the earth.

As it approaches the target area, the water becomes viscous.

"Watch out!" Nana suddenly said, "There's a moving object thirty meters ahead."

The image zooms in. A dark shadowy shape moves irregularly and extremely fast near the cave entrance.

“Not just one,” Carl said. “At least a dozen.”

“That’s when the underwater drone was lost,” Susan said in a low voice.

"Maintain formation," Chen Hao ordered. "Back to back, don't scatter."

They slowly approached.

The entrance was smaller than expected, about two meters high and less than three meters wide, partially obscured by a layer of seaweed-like material. The dark figures hovered around it, sometimes near, sometimes far.

Suddenly, a creature rushed toward the camera.

It resembles an eel, but its body is covered with a grayish-white carapace. Its head has no eyes, only a ring of fine holes. The tail is forked at the end, like pincers.

It opened its mouth and bit into the wreckage of the submarine, tearing off a piece of metal in one bite, chewing it a few times, and spitting it out.

"Eat metal?" Karl's voice tightened.

"Let's try something else." Chen Hao gently pushed the thruster forward, moving it forward half a meter.

The thing immediately turned around and charged at him.

"Defense activated!" Nana shouted.

A flash of blue light appeared on the surface of the diving suit. The creature convulsed violently the moment it collided with the suit, tumbling backward.

The other creatures immediately surrounded them, but did not attack again; they simply swam in circles, maintaining a certain distance.

“Microcurrents are effective,” Nana said. “They are afraid of electricity.”

“But there are too many,” Susan cautioned. “The barrier won’t last long.”

"No need to hold on." Chen Hao stared at the cave entrance. "We just need to rush in."

“I’ll count to three,” Carl said. “Three, two—”

“Wait a minute.” Susan suddenly raised her hand. “There’s a gap in the rock crevice on the left, a little wider than the one in front. It might be easier to get in.”

The camera pans. The gap is indeed larger, and there are currently no creatures guarding it.

"Why is nobody looking over there?" Chen Hao asked.

“It might be too narrow,” Nana analyzed. “It’s not suitable for large creatures to enter or exit.”

"Whatever suits us is fine." Chen Hao adjusted the direction. "Let's go that way."

The four moved slowly.

The biota began to stir. Several rushed forward, only to be repelled by the ionization barrier. The rest emitted low-frequency vibrations, as if transmitting signals.

“They are communicating,” Susan said.

"Whatever they say!" Chen Hao sped up. "Charge!"

They simultaneously activated their thrusters and charged towards the crevice in the rock.

Three creatures lunged at her. Nana instantly increased the current output, and the three bodies froze on the spot, sinking to the bottom of the sea.

The remaining humans took the opportunity to pass through the narrow passage.

A crashing sound came from behind. The carapace struck the alloy shell, clanging loudly. But no one followed.

The passage is very short, only about ten meters long before it ends.

Suddenly, everything became clear.

A circular platform extends outwards, its surface flat, making it impossible to tell whether it is natural or man-made. Vertical grooves appear on the walls, like traces left by some kind of track.

The four people stood still and looked around.

"The barrier is closed," Nana said. "The external threat is gone."

"Oxygen levels are normal," Carl checked the gauges. "We can last another four hours."

“Navigation markers have been established,” Nana added. “Return path locked.”

Chen Hao took off his helmet visor and took a breath of air from the cave.

There was no odor, and the temperature was slightly higher than the outside temperature.

“This place…” he began.

Before he could finish speaking, the ground beneath his feet trembled slightly.

Everyone, please remain silent.

The vibrations came from deep within, very subtle, but continuous, like some kind of machine in operation.

“It’s not an earthquake.” Susan listened intently to the ground. “The rhythm is steady, once every seven seconds.”

“Like a heartbeat,” Carl said.

"It's like a countdown." Chen Hao put his helmet back on.

They took a few steps forward, and the light shone into the depths of the tunnel.

The ground became increasingly clean, with almost no sediment. The trenches on both sides became deeper and more densely packed.

Suddenly, Nana stopped.

"A weak signal was detected."

What type?

“It’s consistent with the last underwater pulse,” she said. “Original encoding, continuously transmitted.”

Chen Hao looked towards the end of the tunnel.

It was pitch black there; you couldn't see anything.

But he knew something was waiting down there.

Wait for them to go in.

Wait for them to open it.