I'm doing engineering in the instance.

The sound of stepping down the first step of the north staircase was quieter than he had expected.

As the support nodes of a cantilever structure age, the deformation during foot traffic absorbs some of the impact energy, much like a spring, resulting in less noise compared to a rigid structure—a counterintuitive principle in engineering. He conducted testing on aging bridges in Bangladesh, using this very principle: listening to the attenuation characteristics of foot traffic to determine the degree of aging of the support nodes. Now, he's using it in reverse, using the degree of aging to predict the noise level.

He walked along the inner side of the staircase, keeping his weight close to the support, step by step, lightly touching the sidewall with his fingers to feel the direction and amplitude of the vibration after each step. By the fifth step, the vibration began to shift slightly outward, so he shifted his weight inward by about half a foot's width. By the eighth step, the structure was stable, the outward tilting tendency disappeared, and the remaining steps could be walked normally.

It took him about two minutes to climb the twelve steps and reach the entrance to the second-floor corridor.

The second floor was even darker, with the roof opening on the north side. Natural light mainly shone on the northern section, leaving a completely shadowed area in the middle of the corridor. He stood at the entrance, waiting for his eyes to adjust to the light before looking inside.

Huh? There's someone over there!

The man was huddled behind a pillar in the deepest part of the shadows, his arms wrapped around his knees, his back pressed against the pillar. His clothes were dark, stained with mud and dust, and his head was bowed low. Xie Chengzhou observed him for about three seconds and judged: male, in his twenties, who had been there for quite some time—judging from his posture, it wasn't a temporary one, but an angle his muscles had adapted to after long-term maintenance; the degree of bending in his legs and the curvature of his back were relaxed, not tense. He was waiting, not preparing to act, but waiting for something to end on its own.

Xie Chengzhou didn't speak. He shifted his gaze from the person and began scanning the second-floor corridor.

The corridor is approximately three meters wide and twenty meters long, with several doors on both sides, most of which are rusted shut. A diagonal crack runs along the floor, extending eastward from the middle of the corridor. The crack is two to three millimeters wide and runs perpendicular to the direction of stress on the floor slab—this is caused by bending deformation, not shearing. The height difference between the floor slabs on either side of the crack is visually estimated to be less than one millimeter, indicating that the deformation has largely stabilized and is not unstable. However, stepping on it produces a loud sound; the edges of the crack are hollow, and the cavities resonate noticeably when stepped on.

He marked the location of the crack in his memo, drew its path, and noted "Detour, high risk of footsteps."

The person didn't move, nor did they look up at him.

Xie Chengzhou began moving deeper into the corridor, avoiding the cracked area and walking close to the west wall. He walked about five meters, then stopped about three meters to the side of the person, glancing at the door on the west side—there was a ray of light through the crack in the door, not a lamp, but an indicator light for some kind of equipment. However, this door didn't correspond to a specific functional area on the first floor; it was probably an auxiliary equipment room. He noted this detail in his memo.

Then he heard a voice.

It was very soft, but he heard it.

It was the sound of shoes rubbing against the floorboards, not stomping, but sliding, like someone was moving their feet. He immediately turned his head—

The person stood up, perhaps because their legs were numb, or perhaps because they saw Xie Chengzhou moving and determined they could move, and began to incline deeper into the corridor, following the central axis of the corridor—

That's exactly where the crack is headed.

Xie Chengzhou took two steps, reached out, grabbed the man's wrist, and pulled him towards his side.

The force wasn't great, but the person clearly hadn't expected it. They shifted half a step to the side, landing on the west side of the crack, not on top of it.

But that half-step produced a sound.

It wasn't a very loud sound, but in this corridor, in this space, Xie Chengzhou calculated its magnitude within two seconds: approximately 60% of the sound of normal footsteps, with a propagation distance of about seven to eight meters. The factory supervisor was currently in the north corridor on the first floor, about 3.5 meters vertically from the second-floor ground. With the sound insulation and attenuation of the floor slab, the actual sound intensity reaching the supervisor's location was about one-third to one-quarter of the original sound.

One-third to one-quarter of the sound should be below the auditory perception threshold of the factory supervisor under normal conditions.

But he wasn't sure.

He pushed the man toward the nearest pillar, gestured for him to stop, and then placed his hand on the ground to feel the vibration.

three seconds.

Five seconds.

The low-frequency vibration rhythm on the first floor remained unchanged, still one step every two seconds, with no shift in direction or change in speed.

Normal state.

谢承洲站起来，在备忘录里记：「二楼走廊意外声音事件。估算传播距离：7-8米，到达厂监位置约为原始强度的1/3-1/4。结果：厂监未反应。阈值边界数据：有效。」

Then, he sensed something different.

It wasn't a sound. It was a vibration under my feet.

The rhythm of the low-frequency vibrations on the first floor suddenly experienced an irregular interruption, then restarted—but at a different speed. Faster. About half again as fast, from two seconds per step to nearly one second per step, and the direction shifted; it was no longer the regular circular patrol route, but a straight acceleration, heading south.

A state of alertness.

Xie Chengzhou put his hand back on the ground to determine the direction of the vibration source: due south-southwest, about fifteen to twenty meters away, moving towards the chemical area.

Then he heard that voice.

The sound came from the first floor, and although it was attenuated by the floorboards, he could still hear it—it was footsteps, not the factory supervisor's, but another kind of footsteps, rapid, the sound of leather or rubber soles on the concrete floor, and uncontrolled, with uneven intervals between the steps, as if they were running.

They started running around in the factory.

Xie Chengzhou closed his eyes, pressed his palms firmly against the ground, and precisely sensed the direction of the vibration.

There was another person on the first floor besides him. As he went upstairs, the other person also began to move, and chose the wrong path—his current position was judged to be either at the edge of the chemical area or already inside. The floor coating in the chemical area had a slightly granular texture, and the sound of footsteps would be significantly amplified by reverberation in this area, as he had verified in Chapter Two. The amplification factor was approximately twice, meaning that the sound of each step this person took was perceived by the factory supervisor as being twice as loud as running in a normal area.

Then he felt a second change in the ground shaking.

More intense, faster, and in a different direction—the factory supervisors were no longer patrolling at an accelerated pace, but tracking in a straight line. The rhythm of the vibrations changed from uniform to impactful, with extremely short intervals between each step, and the entire floor was slightly transmitting the vibrations.

Tracking status.

He pushed the man's hand away slightly and silently made a downward pressing gesture: Don't move.

The person still didn't understand, and a puzzled expression remained on their face.

Then a shout came from the first floor.

It was very short, cut off by something, less than a second, but the frequency of that sound—Xie Chengzhou's muscles contracted slightly in that instant. It was a familiar frequency, not a scream of fear, but one of pain, sudden, and without time to continue. He pressed his palm against the ground, his palm slightly damp. He didn't think about what it was; he was listening.

Then came silence.

It wasn't just ordinary silence; all sounds on the first floor stopped, even the factory supervisor's footsteps ceased, stopping at a certain point. The vibration at that point lasted for about three seconds, stationary and unmoving. During those three seconds, Xie Chengzhou's breath hitched; he held his breath in his chest, waiting for the vibration to make its next move.

Then it was restored.

Two seconds per step, a fixed rhythm, the factory supervisor re-enters patrol mode, the direction begins to shift northward, returning to the normal route.

Xie Chengzhou did not move.

His hands remained pressed against the ground, his palms feeling the return to normal rhythm: two seconds, two seconds, two seconds. Even, calm, like a clock, as if nothing had happened.

He didn't know the person's name. While he was downstairs, Xie Chengzhou hadn't noticed the other person—there were several hiding areas on the first floor of the factory, and he had only observed the factory supervisor's route, not how many people were in the space. He didn't know when the other person entered, which route they took, or what judgment they made. All he knew was that the person chose the direction of the chemical area, started running at a certain moment, triggered tracking, then there was that shout, and then silence.

This is the kind of logic he has seen during the twelve-year project.

It wasn't bad luck, but rather choosing the wrong premise—the premise being "if you don't move, you won't be detected." In most cases, this premise holds true; but in this case, the factory supervisors relied on auditory perception, and the safety boundary for stillness was the sound threshold, not the displacement threshold. The premise changed, and all inferences made under the old premise collapsed simultaneously. They simply didn't have time to verify this.

Hsieh Cheng-chou created a new page in the memo and wrote: "First floor triggered event - unknown personnel. In the direction of the chemical area, running sound was confirmed to trigger tracking status. The plant monitoring system took about 8-10 seconds to lock on. There are no cases of tracking status being withdrawn after it was activated. Recording time: about 12 minutes before the route change."

He paused for a moment.

Then a line was added below: "Incorrect premise. It's not that it's not fast enough, it's that the premise is wrong."

The person in the corridor remained silent. Xie Chengzhou stood up and glanced at him.

The other man's face was pale, his eyes were wide open, and his lips were trembling slightly, but he didn't make a sound or move. He remembered the "don't move" gesture and remained still. Xie Chengzhou glanced at his hands—his knuckles were a little white, from gripping them too tightly.

Xie Chengzhou didn't speak immediately. He glanced at his watch to confirm the time: there were still about six minutes until the next route change. He needed to wait for the route to change, and only after the new route deviated from the northern section could he go to the third floor through the window.

He recalculated the plan: a six-minute wait, plus the safety window after the route change, skipping the second-floor auxiliary equipment room, directly confirming the functional signs and the third-floor entrance location, and then withdrawing before the window closed. This plan could have been executed more leniently, but because of the recent incident, he needed to reassess the factory supervisor's current position—after the tracking incident ended, the supervisor paused on the south side for three seconds, then began to shift north, meaning the supervisor's current patrol starting point was further south, and coverage of the northern area would begin later than the previous round, which actually gave him a few more minutes.

He wrote this update into his memo: "Route change prediction window: approximately 22-25 minutes (factory monitoring starting point shift, north side exposure time delay). Going to the third floor: feasible. Sufficient."

Then he started moving towards the north end of the corridor to look for the function signs.

That person caught up.

His footsteps were still light, landing where Xie Chengzhou had stepped, avoiding the crack in the middle of the corridor, avoiding everything he saw Xie Chengzhou had avoided. Xie Chengzhou didn't stop him, nor did he wait for him, but simply walked forward at his own pace.

On the wall at the north end of the corridor was a sign, made of iron, with most of the paint peeling off, but the lettering was still there. Xie Chengzhou leaned closer to take a look: "Third Floor · Control Area · A/B/C", with an arrow pointing upwards and turning right. This matched his prediction; the control room was on the north side of the third floor, on the right side of the corridor, accessible by continuing up the north staircase.

He updated this information to his memo: "Third floor control room: North side, turn right at the stairs, sign confirmed."

The person stopped beside him and glanced at the sign. After a few seconds of silence, they spoke, their voice low and deliberately suppressed:

"That person down there..."

He didn't finish speaking.

"I can't find out his name," Xie Chengzhou said, "and there's nothing I can do about it."

"I know," the man said. His voice was calmer than before, but the calmness was unnatural, the kind of calm that came from deliberately suppressing something. "I just... I always thought that as long as I didn't move, everything would be fine. I kept hiding, I didn't move at all."

"Factory monitoring relies on auditory perception," Xie Chengzhou said. "It's not vision, it's not movement. You can remain still, but as long as you make a sound, it can hear you."

"So...it's because he got involved in the chemicals business?"

"Sound is amplified in chemical areas," Xie Chengzhou said. "I've verified it. About twice as much. The sound of running already exceeds its perception threshold, and multiplying it by two makes it inevitable to trigger tracking."

The person remained silent for a while.

"How did you..." He paused, choosing his words carefully, "How did you come up with the idea of ​​verifying these things?"

“Without verification, you won’t know where the boundary is,” Xie Chengzhou said. “Without knowing where the boundary is, you can’t act. You can only wait.” He paused. “While waiting, you think you’re staying safe. In reality, you have no data at all; you’re just substituting ‘currently not dead’ for ‘currently safe.’ These are two different things.”

The man didn't answer immediately. His eyes changed slightly, not with sudden enlightenment, but with the kind of change that came from something previously thought to be fixed loosening. Xie Chengzhou had seen this expression before, on construction sites, when a construction team first understood why the construction sequence needed to be changed, or when a young technician first understood the data in a geological survey report—not suddenly finding it simple, but suddenly realizing that they had been thinking about this matter in the wrong way before.

"You've verified it," the person said finally, his tone not asking, but confirming.

"Verified."

The route change occurred approximately five minutes later.

Xie Chengzhou felt a brief interruption in the low-frequency rhythm under his feet, followed by a restart, but the direction had changed—the source of the vibration shifted from south-southwest to due north-east, indicating that the new route for factory monitoring had moved the coverage center to the northeast corner of the factory building, and the area around the north staircase was now on the periphery of the new route, rather than the core route.

It was even better than he had predicted.

He updated the new route on the memo sketch, recalculated the time it would take to get from the current location to the north stairwell and then to the third floor, and calculated the safety window by comparing it with the coverage area of ​​the current route change.

Conclusion: From now on, there is a window of about 25 minutes to go up to the third floor, provided that activation and evacuation are completed before the next route change by the factory supervisor.

He wrote the number in his memo, then added a line below it: "Enough."

Then he stood up, put the memo back in his waist bag, and began to move toward the north stairwell.

The man followed. When Xie Chengzhou passed the crack in the middle of the corridor, he didn't stop or warn him; he simply went around it. The man, watching his own feet, avoided the crack and all the places Xie Chengzhou had avoided. He hadn't learned the actions, but the logic—he had observed Xie Chengzhou's detour last time, and now he knew why.

Xie Chengzhou didn't say anything.

The tremors on the first floor were still even, occurring every two seconds, calm and as if nothing had happened.