I'm doing engineering in the instance.

The wind at the top of the dam is horizontal.

It wasn't pressing down from above, but pushing from the side and blowing from upstream, creating tiny ripples in the water and making Xie Chengzhou's work clothes cling tightly to the left side of his body.

The three of them walked downstream along the top of the dam, with Xie Chengzhou in front, Lao Zhao in the middle, and Li Gong behind.

Xie Chengzhou did not walk fast.

He's watching.

The road on the top of the dam is about four meters wide, with a guardrail on each side. Old Zhao had said that the guardrails were hollow and rusted through, so you shouldn't lean against them. Xie Chengzhou didn't lean against them. He walked in the middle of the road, focusing his gaze on the road and the dam shoulders on both sides. Every few steps, he would stop and squat down to examine a detail.

First location: Road surface crack, sloping direction, about 15 millimeters deep, with smooth edges.

In his memo, he wrote: "Crack ①, oblique, 15mm deep, smooth edges, man-made."

He paused for a moment and added a line: "Direction of force: from top to bottom, about 20° to the left. Tool: conical, about 8mm in diameter, presumably a steel chisel."

The second location: On the side of the dam shoulder, about 30 centimeters from the road surface, there is a section of concrete that has peeled off. The peeled area is about the size of a palm, and the edges are neat.

He squatted down and touched the peeling surface with his hand.

It's new. The concrete cross-section still has sharp edges and hasn't been rounded by water polishing, indicating that the spalling occurred no more than a month ago, possibly even less.

In his memo, he wrote: "Concrete spalling ② • Dam shoulder side • Fresh spalling surface • Edges preserved • Time: Recent • Location: 30cm from the road surface • Tool marks: See below."

He moved the flashlight closer, flattened the beam, and shone it on the peeling surface.

It has lines.

It's not a crack, it's a straight line, parallel, about two millimeters apart, it's a scratch left by a tool on the concrete, and it's horizontal.

He knew what those scratches were.

Chisel.

Using a chisel to cut horizontally, then hammering, the concrete will peel off along the predetermined line, leaving a clean surface without random cracks. This is standard operating procedure in demolition work, used to handle concrete surfaces that require precise removal.

He added a line to his memo: "Tools confirmed: chisel + hammer, horizontal cutting, precise removal, and standard operating procedures."

He stood up.

Old Zhao was beside him, not saying a word, just looking at the peeling surface, touching it with his hand, then pulling his hand back and patting his pants.

"Clean," Old Zhao said.

Xie Chengzhou knew what he meant.

It's not about the concrete being clean, but about the cleanliness of the workmanship. Anyone who's worked on a construction site can tell at a glance—this wasn't just anyone who came to smash it; someone knew how to do it, and it was done very cleanly.

"Have you ever seen this technique before?" Xie Chengzhou asked.

"I've seen it," Old Zhao said. "When they were building the subway, they demolished old structures, that's what they did."

Xie Chengzhou nodded.

They continued walking.

The third point of failure is in the middle section of the dam crest. This one is more obvious – there is a transverse crack in the road surface, about three millimeters wide. He couldn’t directly measure the depth, but judging from the displacement on both sides of the crack, it is at least twenty millimeters, which is already within the scope of a structural crack.

He crouched down and shone his flashlight along the edge of the crack.

There is water in the crack.

It wasn't water seeping in; it was seeping out from the inside. The water marks are from bottom to top, indicating that the crack has penetrated into the seepage pressure zone inside the dam.

In his memo, he wrote: "Crack ③, horizontal, 3mm wide, displacement >20mm, structural, seepage direction: from inside to outside, has penetrated the seepage pressure zone, high risk."

Then he paused for a long time after that line of text.

He was observing the direction of the crack.

The crack is horizontal, extending from the road surface on the top of the dam to both sides of the dam abutment, forming a complete transverse cut. If this crack continues to extend, it will separate the road surface on the top of the dam from the dam body, then the concrete panel on top of the dam body, and finally the internal impermeable layer.

This is the type of damage that dams fear most.

It wasn't water pressure, nor an earthquake; it was the internal damage to the seepage barrier that allowed water to seep into the dam through the cracks, and then the water pressure caused the dam to crack from the inside.

This crack was deliberately designed.

He stood up and mentally reviewed the points of damage.

Crack ①: Slanted, applied force 20° to the left, 15mm deep, depth precisely controlled.

Peeling ②: On the side of the dam shoulder, make a horizontal cut and precisely peel it off to expose the edge of the internal seepage barrier layer.

Crack ③: Horizontal, penetrating the seepage pressure zone, high risk.

There was also a man-made crack in the corridor, which he didn't address.

He mentally connected the locations of the four points of destruction.

It's not random.

These four points of failure form a combination—each alone is not enough to cause the dam to fail immediately, but together they point to three key nodes in the dam's seepage control system: the corridor seepage pressure control section, the edge of the dam shoulder seepage control layer, and the lateral integrity of the dam crest.

This is a complete sabotage plan.

Someone is systematically dismantling the dam's seepage prevention system, not to make it collapse immediately, but to make it fail under specific conditions—for example, when the water level exceeds the warning line.

Xie Chengzhou felt the ground beneath his feet.

The concrete road surface is of unknown thickness, and below it is the dam body, inside which water is seeping.

He wrote a line in his memo: "Inference: The sabotage plan is systematic. Target: Three key nodes of the seepage prevention system. Triggering condition: Water level exceeds the warning line. This is not sabotage, this is design."

Engineer Li squatted down next to him, touched the edge of crack ③ with his hand, and then withdrew his hand.

"I've worked in water conservancy for twenty years," he said. "If I had to choose the location of this crack, I would choose this spot too."

Xie Chengzhou glanced at him.

"Because this is the area where the lateral tensile stress is most concentrated on the dam crest," said Engineer Li. "Temperature changes will cause cracks to appear here first. If another cut is added artificially, the cracking speed will increase by three to five times."

He paused, bent his left ring finger halfway, stopped, and then extended it.

"People who do this understand water conservancy," he said.

Xie Chengzhou remained silent.

He mentally combined that statement with his own deduction.

Systemic destruction tactics. Background in water resources. Precise use of tools.

This wasn't something a player randomly entered the instance and did casually; it was something someone came in specifically to do, with tools, a plan, and a purpose.

"When did they do it?" Old Zhao asked.

Xie Chengzhou mentally compared the freshness of the various points of damage.

The smooth edges of crack ① indicate that it has been there for a while, with rainwater having worn away the edges, estimated to be two to three weeks. The fresh cross-section of the spalling ②, with its angular edges preserved, is estimated to be within the last week. The water seepage from crack ③ is active, indicating that it has been expanding recently, possibly the most recent instance, within the last few days.

"It was done in batches," Xie Chengzhou said. "It wasn't done all at once; it was done in multiple visits, one section at a time."

Old Zhao made a low sound, not speaking, but indicating "I understand".

Xie Chengzhou continued walking forward.

They walked to the middle section of the dam, slightly downstream, where Xie Chengzhou stopped and shone his flashlight toward the side of the dam shoulder.

He saw a mark.

It's not a word, it's a symbol.

A circle with a horizontal line inside, slightly to the left. The circle is about five centimeters in diameter. It was carved with something—not paint, but engraved into concrete. It's a thin line, about one millimeter deep.

Xie Chengzhou recognized this symbol.

This is a "section mark" in engineering drawings, used to indicate the location of structural sections. It usually appears on the elevation view of the construction drawing and is marked "cut from here".

Some people have applied the annotation methods used on construction drawings to the actual structure.

In his memo, he wrote: "Section mark - downstream of the middle section of the dam crest - carved into the concrete - 1mm deep - symbol: engineering drawing section mark symbol - meaning: this is the location of the section - inference: the vandal used engineering drawing language to mark the site - professional background: experience in engineering design or construction drawing."

He crouched down, held the flashlight close to the mark, and flattened the beam.

The markings are even, and the depth of the engravings is consistent. They were engraved with a metal engraving knife, not just drawn randomly. They were measured with tools.

"I've seen this symbol on the blueprints before," said Engineer Li, who was squatting down next to him.

"Cross-section markings," Xie Chengzhou said.

"Yes," said Engineer Li, "but there are no blueprints here. Who is he marking them for?"

Xie Chengzhou pondered the question for a while.

Who is this sign for?

These markings aren't for others to see; they're for yourself. It's a habit—engineers, when conducting on-site surveys, will use familiar symbols to make markings on the site to facilitate quick location on subsequent visits.

This person has come more than once, and plans to come again.

He stood up and wrote this conclusion in his memo: "Cross-section marking function: Self-use positioning marking · Inference: The saboteurs plan to enter multiple times · Next entry target: This cross-section location · Priority: High."

Just as he stood up, he heard a sound.

Rhythmic sounds.

The sound emanating from inside the dam is low-frequency, uniform, and occurs approximately every two seconds.

He planted his foot firmly and felt the direction.

It didn't come from the passage; it came from inside the dam, located below them, roughly in the seepage corridor area in the middle section of the dam.

The seepage victims are still there.

And there's more than one.

He listened for a few seconds and counted the frequency of the rhythmic sounds.

The two frequencies overlapped: one was the one they had encountered in the corridor earlier, at about 0.5 Hz; the other was lower, at about 0.3 Hz, and was one he had never heard before.

two.

In his memo, he wrote: "Seepage creatures - Number update: ≥2 - Frequency ①: 0.5Hz (corridor individuals) - Frequency ②: 0.3Hz (new individuals, larger in size) - Location: Inside the dam - Current distance: Estimated >30 meters."

Old Zhao already had the thermos in his hand.

Xie Chengzhou didn't speak. He turned his gaze from the dam body back to the road surface on the top of the dam and reviewed the current situation.

Water level: 5.58 meters, 2.42 meters from the warning line.

Time window: Approximately 180 minutes remaining.

Seepage: ≥2, inside the dam body, current distance >30 meters.

Points of damage: at least four, with three key nodes of the seepage prevention system already damaged.

They need to do one thing.

It's not about escaping, it's about cultivating.

He put this conclusion in his mind for a moment, feeling its weight.

Build a dam in the instance.

What to use to repair it, how to repair it, and whether the repair would be effective—he didn't know the answers, but he knew one thing: the man-made crack in the corridor that he hadn't dealt with was the only problem they could currently address, because he had seen the crack clearly, knew its depth, its direction, and the location of the triggering device.

He turned to Engineer Li.

"If we temporarily seal the cracks in the corridor," he said, "how will the seepage pressure change?"

Engineer Li did not answer immediately.

He did some mental calculations for a while, then said, "If one section of the corridor's seepage pressure is blocked, the seepage pressure will redistribute in the short term, shifting to other nodes, but the total pressure will decrease because one seepage point has been eliminated."

Will those who infiltrate be affected?

Engineer Li glanced at him. "Do you think the seepage is related to seepage pressure?"

"I'm not sure," Xie Chengzhou said, "but they call them seepage beings, nothing else."

Li Gong paused for a moment.

"It's possible," he said. "If the behavior of the seepers is related to the distribution of seepage pressure, sealing off a seepage point would change their distribution area."

Xie Chengzhou wrote this inference in his memo: "Hypothesis: The behavior of seepage participants is related to the seepage pressure distribution. Verification method: Temporarily seal the cracks in the corridor and observe the changes in the rhythmic sounds of the seepage participants. Risk: Re-entering the corridor."

He glanced at the water level figures.

5.61 m.

Distance from the warning line: 2.39 meters.

The water level is still rising.

"The corridor," he said.

Old Zhao didn't say anything.

He twisted the lid of the thermos to tighten it, then took the cup back into his hand, his thumb gripping the lid and the other four fingers supporting the body of the cup.

This gesture means "I'm ready".

Xie Chengzhou mentally went through the route of the corridor.

Descending from the top of the dam, passing the P2-07 maintenance room, and entering the corridor, he noticed that the seepage device in the corridor had its front end pressed against the door crack when it was closed last time; he wasn't sure if it was still there. The man-made crack was located in the middle section of the corridor, slightly north, about forty meters from the maintenance room. The triggering device was next to the crack; he had seen it clearly last time—it was a loose concrete slab. Stepping on it would trigger the mechanism, while not stepping on it would prevent it from doing so.

They need to temporarily seal the crack.

What should be used to seal it?

He swept over all the visible materials on the top of the dam. The guardrails were hollow and rusted through, unusable. There were a few pieces of concrete debris on the road surface, but sealing cracks in concrete requires mortar; without mortar, there's no bonding force, it only seals the surface, and seepage pressure will bypass it.

He needs a material that can solidify quickly and has a certain degree of adhesion.

He thought about it for a while.

There is water in Lao Zhao's thermos.

It wasn't a water issue; he was wondering if there were any leftover construction materials in the maintenance room in the corridor.

He remembered there was a tool rack in the maintenance room, with tools on it. He glanced at it but didn't look closely. Construction materials weren't usually kept on the tool rack, but there was one thing that might be there—quick-drying cement. It was a temporary sealing material for repairing corridors, packaged in small bags, that could solidify quickly and didn't require mortar mixing.

He wasn't sure, but it was worth a look.

They went down from the top of the dam.

The door to the P2-07 maintenance room was closed. Xie Chengzhou pressed his ear close to the door panel and listened for a few seconds.

There was no rhythmic sound.

He pushed the door open and swept the area with his flashlight.

The maintenance room was in the same condition as when they left: tool racks, water level display, and metal sealing plates for the expansion joint entrances on the walls. The sound of seeping water was still there, dripping, drop by drop, from the walls onto the floor.

He walked over to the tool rack and looked at everything on it.

A wrench, pipe wrench, rubber pad, a roll of wire, two steel rods, a plastic bucket with writing on the lid.

He shone his flashlight over there.

"Quick-drying repair mortar".

Xie Chengzhou picked up the bucket, shook it, and found it to be heavy, indicating it hadn't been opened.

He wrote a line in his memo, then crossed it out and rewrote it: "Quick-drying repair mortar, unopened, can be used for temporary sealing of cracks, setting time: approximately 15 minutes, effective time: to be observed."

The crossed-out line reads: "Luck".

He didn't believe in luck, but he noted this detail down.

The three of them entered the corridor.

Xie Chengzhou was in front, Lao Zhao was in the middle, and Engineer Li was behind. Engineer Li used his left hand to support his right hand, protecting his two sluggish fingers from touching the corridor wall.

The rhythmic sounds in the corridor disappeared.

It wasn't that the Infiltrators were gone; rather, their frequency had dropped below the threshold of Xie Chengzhou's perception. He felt a slight vibration under his feet, very low, about 0.1 Hz, the frequency of the larger individual.

It knew they had come in.

Xie Chengzhou didn't stop. He continued walking forward with even steps, stepping on the dry areas of the corridor floor, bypassing the water accumulation, bypassing the trigger device, and walking to the man-made crack.

The cracks are still there.

It was about two millimeters wide, sloping, extending from the top of the corridor to the side wall, and about sixty centimeters long. The triggering device was about thirty centimeters to the left of the crack; it was a loose concrete slab with tiny cracks on the edges. Stepping on it would cause displacement. He didn't know what the displacement would trigger, but he had made a record of the last activation of the seepage entity and the location of this slab.

He placed the mortar bucket on the ground and pried open the lid.

The mortar is gray, dry, and has a slight chemical odor.

He scooped up a handful, felt the texture, then poured a small amount of water from Lao Zhao's thermos into it and started mixing.

Old Zhao squatted down next to him, watching him mix the mortar, without saying a word.

Engineer Li stood guard at the entrance of the corridor, his flashlight sweeping into the depths of the corridor every few seconds.

Xie Chengzhou mixed the mortar, used a steel shovel to fill the crack with the mortar, starting from the top and pressing down to fill it firmly, and then used the side of the steel shovel to smooth the surface.

He filled it out in about three minutes.

The surface of the crack was covered with mortar, which began to harden and darken in color from gray.

Then he heard the change.

Rhythmic sounds.

The one at 0.5 Hz, the individual in the corridor, its rhythmic sound stopped after they entered the corridor, but it has reappeared now, but the frequency has changed—it's not a uniform 0.5 Hz, but 0.4 Hz, a little slower, and the rhythm has become irregular, as if it's searching for something again.

Then there's the larger one at 0.3 Hz. Its frequency also changed, dropping from 0.3 Hz to 0.25 Hz, then pausing for three seconds, and then starting again at 0.28 Hz.

The rhythms of both individuals have changed.

Xie Chengzhou stopped and pondered the change in his mind for a few seconds.

He filled the crack with mortar, and the rhythm of the seepage changed.

This is no coincidence.

In his memo, he wrote: "Construction behavior (temporary sealing of cracks) → Changes in the rhythm of seepage agents • Direction of change: decreased frequency + irregular rhythm • Inference: Seepage agent behavior is related to seepage pressure distribution • Sealing one seepage point → redistribution of seepage pressure → change in seepage agent behavior pattern • Important: Construction behavior may affect the behavior pattern of threat entities • Further verification is needed."

After he finished writing that line, he closed the memo.

Li Gong said at the entrance of the corridor, "They are moving."

Xie Chengzhou stood up and shone his flashlight deeper into the corridor.

In the light, something was moving deep in the corridor, not towards them, but towards the other end of the corridor, upstream, moving slower than before, and the frequency of the rhythmic sound was decreasing.

They are leaving.

Xie Chengzhou noted this detail down in his memo, then turned his attention back to the sealing surface of the crack.

The mortar is still setting, its color continues to darken, and fine shrinkage lines begin to appear on the surface. This is a normal phenomenon during the setting process of quick-drying mortar and does not affect the sealing effect.

He squatted down, touched the surface of the mortar with his hand, and felt its hardness.

It's still soft, but it's starting to have some support. It will solidify completely in about ten minutes.

He stood up and reviewed the current situation.

Water level: 5.61 meters, 2.39 meters from the warning line.

Seepage makers: Two, moving upstream, behavior pattern changed, cause: construction behavior (temporary sealing).

Crack sealing: In progress, expected to solidify in ten minutes.

Vandal: Professional engineering background, water conservancy or civil engineering, multiple entries, next target: the location of the dam crest section marker.

He added a final line to his memo: "Unresolved issue: The saboteurs' goal isn't to collapse the dam, but to disable it under specific conditions. What conditions do they want to trigger it? Why?"

The sound of water seeping in the corridor was still there, drop by drop, evenly, just like before.

But the rhythmic sound disappeared.

Xie Chengzhou felt the soles of his feet.

The vibration is still there, very low, 0.1 Hz. It's the larger entity that hasn't completely left; it's just retreated to a more distant place.

He noted this detail down in his memo, then added a question mark after that line: "What is it waiting for?"