I'm doing engineering in the instance.

Chapter 4 Workers' Rest Area

After the vibrations from the route change completely stopped, Xie Chengzhou turned a page in his memo.

He was looking for a record he had marked earlier. Early on, upon entering this unknown factory, he noticed an area on the west side of the first floor that differed from the overall industrial layout of the factory—that side had artificially excavated ventilation holes in the wall, and the cement grout on the floor was a slightly different color, finer, and showed more signs of repair, suggesting it was an area requiring regular access. He marked "→West Side · To be Investigated" on the memo sketch, but didn't go in because the factory supervisor was nearby.

The current window is 25 minutes. The coverage focus of the new factory monitoring route is in the northeast corner, while the west side belongs to the lowest frequency area in the new route.

He closed the memo and tightened the clasp of his fanny pack by one notch.

The person who had been following him from the second floor noticed that he had stopped, and stopped as well.

"You stay here," Xie Chengzhou said. "North end, don't go into the crack area, and don't make any noise. I'll go down to the first floor, it'll take about ten minutes."

The person glanced at him but didn't ask what he was doing. He simply nodded.

Xie Chengzhou was almost at the top of the stairs. He placed his hand on the side wall of the stairs to check the vibration status on the first floor. The factory supervisor was in the northeast corner, with a rhythm of one step every two seconds and a stable direction. There was still a considerable distance between them and the north stairwell area.

He started to go downstairs.

Going downstairs took about half the time compared to going upstairs. He knew the route well; he knew which steps to keep to the inside and which steps to lower his center of gravity for. His muscle memory had been established. He firmly planted his foot on the first floor, removed his hand from the side wall of the staircase, and stood at the entrance of the corridor to double-check the direction of the vibration.

Factory monitoring station, northeast corner, has not been moved.

it is good.

He walked westward, hugging the south wall of the inner corridor of the factory, bypassing the edge of the chemical area. The route had already been marked in the sketch in the memo, so he hardly needed to think about it anymore. The lighting on the west side of the first floor was even worse than in the central area. There were no openings in the roof, and the natural light came from a few ventilation holes in the west wall. The thin beams of light slanted in at a low angle, illuminating only a section of the floor, leaving the rest in shadow.

He stepped into the shadows and began to adjust his eyes.

The workers' rest area was right next to the ventilation opening, which was basically the same location he had predicted.

Despite being called a "rest area," it was actually just a corner partitioned off on the west side of the factory: between the corners of two walls, there was a long wooden table with most of the paint peeling off the tabletop, but the legs were still sturdy and not obviously deformed; on the table was an enamel mug, upside down with its opening facing down, as if it had been used for the last time and then just left there, never to be picked up again; on the wall was a safety production slogan, the paper was yellowed and partially torn, but the words were still legible: "Safety Production - Everyone's Responsibility," the font was hand-painted, the strokes were somewhat uneven, and it was not printed.

Xie Chengzhou stood in front of the table for a moment and scanned the entire area.

He had done structural testing for aging and had seen many abandoned industrial sites. He knew what a site looked like when it was last used. The angle at which the enamel mug was upside down, the slight wear marks in the center of the table (from friction from long-term storage), the yellowing of the safety slogan paper—this place wasn't suddenly emptied at a certain point in time; it was gradually taken out of use. When the last person left, he knew he wouldn't come back, so he turned the mug upside down.

Xie Chengzhou marked these details in his memo and then began a systematic search.

He found a second object under the table.

It was a work permit, wedged between the table leg and the wall. It might have slipped in from the tabletop, or someone might have intentionally slipped it there. The hard, blue cover was slightly warped from moisture but hadn't disintegrated. The embossed seal on the cover was still clear: "Employee Work Permit of [Chemical Plant Name]," followed by a handwritten number, the ink faded but still legible. Xie Chengzhou opened it. Inside was a photo and a line of information: Name—a name he didn't recognize; Occupation: "Maintenance Fitter"; Date of Issuance. The year was blurred in both the photo and the information, but from the style of the work clothes and the helmet in the photo, Xie Chengzhou judged that the work permit couldn't be too recent.

He closed his work ID but didn't put it back. He put it into his waist bag.

He continued searching the tabletop and under the table, and found the third object on the west side of the table leg, near the corner of the wall.

It was an iron tool, a slender handle about thirty centimeters long, with a solid conical tip at one end and a flat, angled nozzle at the other. He picked it up and weighed it. It was about four to five hundred grams, made of iron, forged rather than cast. The handle showed slight wear but was not deformed, and the overall structure was intact. He tapped the conical tip lightly against the back of his hand; the sound was crisp.

He knew what it was.

This is a testing hammer, used by a specific trade—it determines whether there are hollow areas inside by tapping the surface of a structure and listening to the changes in the echo. He used a similar tool when conducting foundation testing, specifically to check the internal integrity of concrete piles. The principle is the same: solid and hollow structures have different sound attenuation coefficients, and the echo frequency after tapping can determine the internal condition.

He flipped the testing hammer in his hand and glanced at the wear marks on the flat tip—the wear on the tip was uniform, formed by long-term, regular use, indicating that the previous user of this tool was an experienced worker who used it correctly and with consistent force, not just randomly striking.

He put the testing hammer into his waist bag, then took a step toward the table and habitually placed the tool close to the ground.

This is a habitual action he takes when conducting on-site testing—before starting the tapping test, he first places the tool flat on the ground to feel the ground's vibration benchmark and eliminate external interference. This step is almost meaningless on a normal construction site, as the sources of external vibration are too complex. The significance of this step is a psychological anchor point, not actual data.

But this is not a normal construction site.

Xie Chengzhou pressed the testing hammer onto the ground with the conical head facing down, holding the flat end to ensure complete contact between the tool and the ground.

What he felt was not the same thing he felt when he touched the ground with his hands.

The conductivity between iron tools and concrete is far greater than that between skin and concrete—this is common physics, and he knew it, but he hadn't expected the difference to be so significant. The vibrations transmitted through his hand holding the tool were about two to three times clearer than what he had felt directly with his palm before, with more precise directional discrimination and richer frequency details.

He stood there, focusing all his attention on the vibrations transmitted from the tools.

Factory supervisor, northeast corner, two seconds per step, even rhythm. He already knew this; it was the baseline for normal patrol status.

But he sensed something he hadn't sensed before.

Before the factory supervisor took each step, there was a very subtle vibration—not the footstep itself, but something even earlier, about 0.3 to 0.5 seconds earlier. It was a slight "pre-compression," like the ground receiving a small force when the center of gravity shifted before the full vibration of the step. He hadn't felt this at all when he placed his palm on the ground before, but with the tool, the signal was amplified to a discernible level.

Xie Chengzhou went over this discovery in his mind.

The "pre-compression" signal appears approximately 0.3 to 0.5 seconds before footsteps—if this signal is stable, it means the observer can anticipate the monitor's next landing point and location, with an advance margin of about half a second. On its own, half a second doesn't seem significant for decision-making; however, if this signal can be accurately identified, the observer can use this advance margin to continuously track the monitor's movement, moving beyond passively sensing ground vibrations to actively predicting the monitor's next move.

This means he can perform calculations while in motion, rather than stopping and waiting for vibrations.

He stopped in place, lifted the conical head of the testing hammer from the ground, and wrote in his memo: "Hidden Rule D (Suspicious): Iron tools touching the ground amplify vibration transmission. A 'pre-compression' signal was detected, leading the footsteps by approximately 0.3-0.5 seconds. If stable: the monitoring position can be predicted during movement. To be verified."

Then he looked up at the watch clipped to his fanny pack.

Seven minutes and eighty seconds had passed since he left the second floor, and there were still about seventeen minutes left at the window.

He added a line to his memo: "Current supervisor's location: Northeast corner, away from the west and north stairwell areas. Approximately 17 minutes remaining at the window. Enough."

He stood up, adjusted the position of the detection hammer in his waist pack, slightly altering the weight distribution. He recalculated the counterweight on the other side to ensure it wouldn't affect his center of gravity control during movement. Then he started walking eastward, preparing to return to the north staircase.

He walked about ten steps and then stopped.

It wasn't because he heard anything, but because he thought of something, something that wasn't directly related to the current task.

That work ID.

He took it out, reopened it, glanced at the photo, and then at the blurry information. The name, the number, the yellowish face. This person had worked here, used that testing hammer, developed the habit of pressing it to the ground before using it, then tucked his work ID between the table leg and the wall—whether intentionally or unintentionally, he didn't know—and then left.

He didn't know when the factory was abandoned. He didn't know the circumstances under which the worker left. He didn't know if the copy had any historical background set for this space.

But one thing caught his attention: the way the enamel mug was worn, the way the work ID was handled, the wear marks on the inspection hammer—these details didn't seem like randomly generated props. The details of props exist for their function, and these details went beyond what was required for function—the angle at which the enamel mug was upside down, the spot where the work ID was tucked in—these were details left by a real user, not designed to be discovered, but the result of natural behavior.

Xie Chengzhou wrote in his memo: "The density of detail in the workers' remains exceeds the reasonable range for props. Hypothesis: The replica space is a capture of real history, not generated out of thin air. If true, then the existence of the factory supervisor, the distribution of chemicals, and the aging of the structure are all real historical conditions. Awaiting cross-replica verification."

He paused, looking at the line of text.

He then closed the memo and continued walking towards the stairwell.

In the second-floor corridor, the player was still at the north end, standing motionless against the wall. Xie Chengzhou went upstairs, and the person looked up, not saying a word, but confirming with his eyes that Xie Chengzhou had returned.

"Seven minutes and forty seconds," Xie Chengzhou said, "twenty seconds faster than I estimated."

The man glanced at him. "What did you find down there?"

"A new tool," Xie Chengzhou said, "and a rule that needs to be verified."

"New rules?"

"Still verifying," Xie Chengzhou said. "It's not important right now." He glanced at his watch. "There are about seventeen minutes left, enough to get to the third floor, enough to complete the activation, and enough to evacuate. Let's go."

The person pushed their back against the wall away, stood up straight, and then followed.

Xie Chengzhou had already walked ahead without looking back. He continued along the west wall, bypassing the crack in the middle of the corridor and all the previously marked risk points. He glanced at his watch one last time, then shifted his attention from the time to the vibrations beneath his feet.

The factory supervisor is in the northeast corner, moving at a steady pace, taking one step every two seconds.

Xie Chengzhou took the testing hammer out of his waist bag, casually held it in his left hand with the conical head facing down, and took a few steps to let the tool lightly touch the ground - not to detect, but to sense, using iron to conduct and amplify the vibration signal, continuously tracking the factory supervisor's position while moving.

The "pre-compression" signal was clear, leading the footsteps by about 0.3 seconds. The monitoring station was located in the northeast corner and did not shift westward.

He added a parenthesis after that line in the memo: "(Under verification - Preliminary stability)".

The north staircase was already ahead. He gripped the testing hammer tighter and began to climb.