The sound is still coming from below.

Chen Hao leaned against the rock wall, his fingers tracing patterns on the recording tablet. The sound wasn't urgent, but it never stopped, like someone banging on an iron pipe in the distance. He looked up at Nana; she was processing data, her mechanical eyes flashing blue.

"The diagram is out?" he asked.

“It’s out.” Nana raised her hand, and a semi-transparent projection appeared in the air. A winding passage unfolded before them, like a crooked water pipe. “The main entrance is three meters below, sloping upwards, and the narrowest point is only eighty centimeters wide.”

"So narrow?" Carl leaned closer. "Even a person couldn't squeeze through."

“No one needs to go in,” Nana said. “We can just bring the things in.”

Susan stared at the image for a few seconds: "What about the water flow speed?"

"The peak value we just measured was 4.7 meters per second, and it has now dropped to 3.1 meters per second. The pressure is decreasing, but it hasn't stopped."

Chen Hao tucked the record board back under his arm: "Then let's get started before it recovers."

Nana nodded: "I've already designed the sealing structure. The outer layer uses titanium alloy plates for pressure resistance, the middle is filled with sealant, and the inner layer is reinforced with cable mesh for toughness. It's like a sandwich."

"Sounds delicious," Carl grinned. "But can this thing even be made?"

“All the materials are here.” Nana pulled up the list. “The titanium plate is from the cutter’s casing, the sealant is in the emergency kit, and the cable is a spare line; just cut a section.”

“Okay.” Chen Hao slapped his thigh and stood up. “Then let’s get started. Susan, keep an eye on the water level and call out immediately if you see anything wrong; Carl and Nana, prepare the components; I’ll go see if I can find a nearby construction site.”

No one objected.

Carl crouched down and began disassembling the toolbox. Susan turned on her terminal and connected to the drone's signal. Nana stood still, but the projection kept changing, with parameters scrolling by line.

Chen Hao walked down the rock face for a while. His feet slipped, and he had to hold onto the wall to steady himself. Ahead was a raised stone platform, about three meters from the main passage entrance, at a suitable angle.

“This works,” he called back. “Let’s start here.”

In less than ten minutes, the first sandwich block was finished. It consisted of two titanium plates sandwiched together with glue, secured around the edges with welds, and reinforced with a steel bar for slip prevention.

“Let’s test it out,” Nana said.

Carl picked up the block and stuffed it into a small crack nearby. The glue began to expand as soon as it came into contact with water, slowly filling the gap. Drone footage showed the water flow slowing down significantly.

“Okay,” Chen Hao said. “Make another one, then we’ll have our main course.”

The second one took even longer. The low temperature made the sealant thick, and Karl almost got his gloves stuck. Nana had to temporarily change the catalyst ratio to get the reaction to proceed normally.

"Alright." He wiped his face. "Both pieces are ready."

"Ready to deploy." Nana started up the last drone, flew it to the front of the main passage, and scattered a small amount of fluorescent powder. Green dots floated in the black water, marking the entrance location.

“I can see it.” Chen Hao fastened his safety rope and began to descend with Karl. The rock surface was slippery, and the two of them inch by inch, clinging to the wall.

Susan shouted from her vantage point, "The water level is rising again! Be careful!"

"I heard you!" Chen Hao replied without stopping.

When they reached the platform, the water had already risen half a meter. The main passageway was just below them, like a black hole, sucking in the surrounding mud and sand.

"Push one in first!" Nana's voice came through the earpiece. "The angle should be aligned with the contraction zone."

Chen Hao and Karl worked together to lift the first sandwich block. It was very heavy, and lifting it was difficult. They moved forward little by little until the edge got into the groove at the opening.

"Hammer!" Chen Hao reached out.

Carl handed over the hydraulic hammer. Two blows later, the titanium plate was embedded in the rock crevice. Just as he breathed a sigh of relief, a surge of backflowing water crashed in, lifting one side of the plate; the adhesive hadn't fully cured yet.

"It's going to fall!" Carl shouted.

Chen Hao immediately pulled out a section of cable, looped it around the edge of the board, and tied the other end to the anchor post. The water flow tightened the cable, pulling the board inward.

"It's done!" he exclaimed, panting.

The second piece went much more smoothly. The two men pushed it forward and secured it in the same way. After the two boards interlocked, the glue in the middle continued to expand, sealing the entire opening tightly.

The water flow immediately subsided.

Susan looked at the monitor above: "The flow rate has dropped by 70%! The pressure is also decreasing."

“There’s still about 30% leaking,” Nana added, “but the direction has changed. Now it’s seeping out from a small crack on the side, not the main spray.”

"That's enough." Chen Hao leaned against the rock wall. "At least it won't all collapse at once."

They retreated back to the platform. Karl slumped to the ground, took off one glove, and shook his hand: "At least it wasn't a wasted effort."

Susan was still recording data: "The water quality is starting to stabilize, and the methane concentration has not increased."

Nana stared at the screen: "I suggest making two more reinforcement pieces to patch the side cracks. Although it's under control now, we can't expect this glue to hold forever."

"We'll talk about it later." Chen Hao waved his hand. "Let me rest for two minutes. This job is much more tiring than carrying bricks."

No one laughed, but the atmosphere relaxed.

A few minutes later, Carl got up: "I'll go collect the remaining titanium plates; maybe we can assemble a third barrier."

"Don't go too far," Susan cautioned. "This rock formation is still unstable."

"I know." He picked up his tool bag and walked towards the ruins.

Chen Hao looked down at his shoes; both were soaked. He didn't change them, nor did he intend to. After all, coming down here wasn't easy, and he might end up wading through water anyway.

Nana suddenly spoke up: "Vibration detected."

"What level?" Susan looked up.

"Mild, lasting three seconds. Source... uncertain."

"Is it caused by the water flow?" Chen Hao asked.

“It doesn’t seem like it.” Nana shook her head. “The frequency is wrong. It’s more like… movement inside the rock mass.”

"Move?" Karl stopped in his tracks.

“I suggest temporarily halting heavy operations,” Nana said. “I’ll decide on the next step after I’ve finished my analysis.”

“Okay.” Chen Hao stood up. “Then hurry up and calculate. We’ll wait here.”

Susan turned off the terminal screen and looked up at the rock wall above her. A thin line extended down from the edge, as if it had been slowly torn apart by something.

She didn't say anything, but simply moved the sampling bottle next to her to the side.

Carl crouched down to check his tool bag and found a wrench was missing. He remembered putting it in just now.

"Who took my wrench?" he asked, turning around.

No one responded.

He got up and walked back to where he had been standing. There was a half-footprint in the mud, not their size.

"Wait a minute." He stopped. "Has someone been here?"

Chen Hao frowned: "Don't joke around."

“I’m not kidding.” Carl pointed to the ground. “These aren’t our markings. The footprints are coming from that direction—” He raised his hand and pointed deep into the vein, “and they were just left.”