The crow perched on the solar panel for a few seconds, then fluttered away. Chen Hao stared in the direction it disappeared, without moving.

Nana's voice came from behind: "The wind speed is 3.7 meters per second, and the grass blades are swaying at an angle of 13 degrees."

“I know,” Chen Hao said. “I’m not just watching the wind.”

He looked down at the soft soil at his feet, squatted down, stuck his finger in, and rubbed it a couple of times. "It's still wet. It didn't rain yesterday."

“The soil moisture remains consistently high,” Nana said as she approached. “It’s noticeably different from the surrounding areas.”

“It’s not seepage.” Carl also squatted down and brushed away the topsoil. “There’s no trace of water underneath.”

Susan, carrying her toolbox, walked over and stopped upon hearing this. "You suspect it's hollow?"

“I wouldn’t dare say,” Karl shook his head, “but when you step on it, there’s a slight rebound, like there’s a cushioning layer underneath.”

Chen Hao stood up and clapped his hands. "Whether there's a hole or not, we have to prepare for the worst."

"You mean...the tunnel?" Susan frowned.

“It’s not about digging out,” Chen Hao pointed to the ground. “It’s about digging down. To build a shelter. If it really collapses, we need somewhere to crawl in.”

Nana's eyelight lit up, and she began retrieving data. "Underground structure scanning in progress. The current depth of three meters is mainly composed of dense sandstone, with some weathered shale interspersed, providing conditions for shallow excavation."

How deep can we dig?

"The recommended safe depth is 4.5 meters, and a supporting structure and ventilation channel must be installed."

What about exports?

"The dual-exit design is more secure. The main exit is located on the west side of the shed, while the backup exit leads to the waste pile in the north area, which is 30 meters away from the wall and is highly concealed."

Chen Hao grinned. "You're pretty good at picking locations."

“This is a standard disaster mitigation engineering model,” Nana said, “but its implementation requires the availability of excavation tools suitable for the geology.”

Her smile froze.

"Tools?" Chen Hao turned his head. "Don't we have shovels?"

“Ordinary shovels are suitable for soft soil.” Nana pulled up a set of comparison pictures. “The current strata contain 36% rock debris, which requires impact or rotary excavation equipment. The existing electric drills are not powerful enough and there are no special drill bits.”

Chen Hao glanced at the toolbox in the corner. Karl had already opened it, pulled out a rusty chisel, and then a worn-out hand drill.

“This thing can’t even drill through wood,” Carl sighed.

Susan put the toolbox on the ground. "This is all we have in stock. The power tool batteries are all dead, and the motors are half burned out."

"What about the nearby ruins?" Chen Hao asked.

“It’s searchable,” Nana said. “But according to the remaining satellite maps, the nearest engineering facilities are seven kilometers away, and most of the buildings have collapsed.”

"I have to go, even if it's seven kilometers." Chen Hao grabbed his backpack. "I can't exactly pick at the rocks with my bare hands."

Half an hour later, the four split into two groups and set off. Chen Hao and Carl headed towards the ruins to the east, while Susan and Nana stayed at the base to inventory the remaining parts.

The ruins were a cluster of low-rise factory buildings with crooked tin roofs and withered vines climbing the walls. Chen Hao stepped forward on a collapsed concrete slab, making a cracking sound under his feet.

"Be careful," Carl tugged at him. "This part is more fragile than it looks."

They first pried open the door of a small warehouse. Inside were several rusty barrels, and in the corner was a small, abandoned drilling rig.

Chen Hao crouched down to inspect. "The motor is gone?"

“The drive shaft is broken.” Carl disassembled the casing. “The gears are jammed, and the oil passages are completely blocked.”

"Can it be used?"

"Unless there is a replacement part."

They continued walking inside. In a drawer of an office, Chen Hao pulled out an instruction manual for a vibratory tamping machine; the cover read "Portable High-Frequency Impactor."

"If this thing were still here..." He looked up.

Under the desk lay a metal casing, covered in dirt and grime. He dragged it out and wiped it clean—the backpack-like frame was intact, the handle was still there, but the motor was burnt and smoking.

"The shell is sturdy enough," Carl said, weighing it in his hand. "It's a pity the inside is ruined."

"Take her back." Chen Hao carried her on his back. "Nana might be able to come up with something interesting."

Meanwhile, Susan and Nana were organizing inventory in the shed. Nana used a scanning arm to scan each tool and mark its usability.

“Thirty-seven screws, twenty-one of which have intact threads,” she counted. “Four sections of steel pipe, varying in length, with wall thickness meeting pressure-bearing standards. One shovel blade, the break is in the middle, the front twenty centimeters can be cut off as a cutting head.”

Susan sorted the parts and laid them out on the floor. "What can we possibly assemble with all this?"

“It cannot be considered a complete tool at present,” Nana said. “It lacks a power source, transmission structure, and control system.”

"Then let's wait for them to come back."

Before evening, the two teams met at the work shed. Chen Hao placed the rammed earth machine casing on the table and brushed off the dust. "Found this."

Carl put down his backpack and took out a gear set from a hydraulic arm. "It's from an engineering vehicle. I don't know if it still works."

Susan spread out the blueprints. "We have steel pipes, screws, and broken shovel blades here. Put them together and it looks like a weapon, not a tool."

Nana began scanning all the objects she had brought back. The optical lens moved back and forth, projecting three-dimensional models.

“Under analysis,” she said. “The original power of the impact ram is 1,200 watts. If the power module is replaced, it can be modified into a handheld excavator. But there is currently no matching motor.”

"Where's the generator?" Chen Hao asked.

"The base's backup generator has an unstable output and is too large to be portable."

"Where's the battery?"

"The highest voltage of the existing battery packs is twelve volts, which is far below the equipment requirements."

Chen Hao plopped down in a folding chair. "So we have a pile of scrap metal, but we can't start construction?"

No one spoke.

The light on the shed ceiling flickered twice, casting a dim, yellowish glow on the table. The parts were scattered haphazardly, like a failed jigsaw puzzle.

Carl picked up the gear and wiped it with his sleeve. "I've seen a manual auger before, which is turned by hand and has a conical drill bit at the bottom that can drill into hard soil."

How deep?

"Two or three meters. It's slow, but it doesn't need electricity."

“We need to dig four and a half meters,” Chen Hao said. “We still have to go through the rock strata. By hand? We wouldn’t finish digging even if we did it next year.”

"Is it possible to use the principle of vibration?" Susan suddenly asked. "Ramming machines break the ground by high-frequency vibration. If we can make this shell vibrate similarly, even with lower power, we can still chip it in little by little."

Nana nodded. "Theoretically, it's feasible. But it requires an oscillation generator. There's no readily available module for that."

"Could you build one?"

"Insufficient materials. Lack of piezoelectric ceramic components and high-frequency drive circuits."

Chen Hao stared at the burnt-out motor and suddenly asked, "Why did it burn out?"

"The overload protection failed, and continuous operation caused the coil to overheat and melt," Nana replied.

"So that means...it could have turned?"

"yes."

What's the RPM?

"Three thousand times per minute."

Chen Hao's eyes lit up. "Three thousand times...isn't that vibration?"

“The logic holds true,” Nana said. “If the motor can be repaired or the core of the vibration can be extracted, it can be used as a simple vibration source.”

“But the motor is broken,” Carl warned.

"It's not necessarily all useless." Susan leaned closer to examine it. "Maybe there are still usable coils inside?"

“But we don’t have any testing equipment.” Chen Hao scratched his head. “We can’t find any intact ones even with a multimeter.”

Nana paused for a few seconds. "I suggest disassembling the motor. I can use a microcurrent probe to test the conductivity of the copper wires segment by segment."

"Then give it a try."

Nana started disassembling the machine. Chen Hao lay on the table, watching the parts being laid out one by one. Susan picked up a piece of enameled wire with tweezers and examined it against the light.

Carl suddenly said, "I remember there's a repair shop in the East District that used to do maintenance for construction teams. We've never been there."

"How far?"

"About four kilometers."

"Let's go check tomorrow," Chen Hao said. "Maybe they have some in stock."

“Only if we can still walk.” Susan looked up. “We only have half a tank of gas left, and the SUV tires are almost worn out.”

“Let’s walk,” Carl said. “It’ll save fuel.”

It was completely dark outside. The newly installed LED strip lights came on, illuminating a small patch of ground in front of the workers' shed.

Chen Hao got up and walked to the door, gazing at the two blue and white lights. A moth bumped into the lampshade, bounced, and fell into the grass.

He turned to look at the table. Nana was still working, her optical lens aimed at the wreckage of the motor, her probe arm gently twirling a thin wire.

Susan laid out several sections of steel pipe side by side and marked the cutting positions with a marker.

Carl sat in the corner, holding the gear in his hand, rubbing the tip of the teeth repeatedly.

"Are we really going to dig a tunnel using all this junk?" Chen Hao sat back in his chair, his voice lowering.

No one answered.

The projector on the table was still spinning, displaying a diagram of the rammed earth machine's internal structure. A red line marked the location of the vibration module, with the words "**Insufficient energy input, unable to activate**" written next to it.

Chen Hao reached out and touched the projector, causing the image to shake.

"If only I had a welding torch," he said.

“Or an angle grinder,” Susan replied.

"And another hydraulic shears," Carl added.

"Why bother modifying these?" Chen Hao said with a wry smile. "We might as well just buy a tunnel boring machine."

Nana looked up. "The current materials are insufficient to restore the standard equipment."

The room fell silent.

“But if an asymmetric reconstruction design is adopted,” she continued, “a simple tunneling device could be assembled.”

"What is asymmetric reconstruction?" Chen Hao asked.

“It’s about not assembling according to the original structure,” Nana explained. “We use existing components and redistribute the functional modules. For example, we use steel pipes as the main support, a vibrating shell to provide power, gear sets to enhance torque transmission, and manual application of rotational force.”

"It sounds like building Lego," Chen Hao said.

“They’re essentially the same,” Nana said, “except the cost of failure is higher.”

Chen Hao stared at the pile of parts and suddenly chuckled. "The four of us—one a slacker, one a robot expert, one an engineer, and one who's repaired tractors—are going to use this pile of scrap metal to build a tunnel-digging machine."

What is the success rate?

"The current estimate is 31%."

"That's pretty high." He leaned back in his chair, looking up. "Higher than my chances of getting into college."

Susan tossed the marker into the pen holder. "I'll go to the East Side repair shop tomorrow."

“I’ll go with you,” Carl said.

“I’ll stay home and watch over the equipment,” Chen Hao said, pointing to Nana. “Let her continue disassembling the motor. What if she finds a good wire?”

Nana's eyelight flickered slightly. "Attempting the thirty-seventh continuity test."

Chen Hao stood up, picked up the bent metal tube, and examined it closely. He walked to the table, placed it on the projection screen, and aligned it precisely with the handle.

“Look,” he said to Nana, “this tube is bent just right so we can grip it tightly. It’s like it was made especially for us.”

Nana glanced at it. "The probability of a coincidence is 6.8 percent."

"Don't be a spoilsport," Chen Hao said. "At times like this, you have to believe in luck."

He put the pipe down and tapped his fingers on the table. "We don't have a factory, a workshop, welding equipment, or blueprints. We can't exactly use our hands to plan it, can we?"

The light in the shed flickered again.

Nana's projection suddenly jumped, and a red line extended from the motor wreckage, connected to the rammed earth machine casing, then to the steel pipe, and finally pointed to the gear set.

A dashed path forms a closed loop.

"We've identified potential combination options," she said. "Further validation is needed."

Chen Hao leaned closer. "Wait, did you think of something?"

Nana pointed her optical lens at him. "Please provide a clean work surface; I need to arrange the components for a physical simulation."

Chen Hao immediately cleared the desktop.

Nana lifted the mechanical arm and placed the burnt motor wreckage in the center, with steel pipes, gears, broken shovel blades, and the rammed earth machine casing arranged in order next to it.

A thin beam of light shot from her fingertips to measure the distance.

“The initial configuration is complete,” she said. “It’s named ‘Simple Vibration Tunneling Unit.’ Prototype assembly will take two hours.”

Chen Hao's eyes widened. "Didn't you just say you didn't have a plan?"

“We didn’t have enough data before,” Nana said. “Now we have new parts.”

Susan and Carl also gathered around.

The scattered scrap metal on the table lay quietly under the projected light.