After the footsteps faded away, Chen Hao immediately raised his hand to signal everyone to stay still. He crouched down against the wall, his ear pointing deep into the passageway.

“Go,” he said. “Turn left.”

The four of them moved along the rock face, the loose stones slipping on their shoes. As he walked, Chen Hao picked up a few small stones from the ground and arranged them into an arrow shape at a corner. After he finished, he straightened up and took a breath.

“This place is like a maze,” Carl said in a low voice.

"It's even more convoluted than the supermarket downstairs from my house." Chen Hao wiped his face. "At least there you can smell the oden there."

Nana walked at the front, her eyes shining faintly. The lighting module flickered, like a faulty light bulb. Every few minutes it would go completely dark for about thirty seconds each time.

The third time it went out, the darkness lasted exceptionally long.

"One, two, three!" Chen Hao suddenly shouted.

No one answered.

He shouted again, "One!"

"Two!" Susan's voice came from the right.

"Three!" Karl was further away.

“Four!” Nana responded.

"I'm behind you," Chen Hao said to Susan. "Don't move."

The ground was slippery; Susan had almost slipped and fallen. Now she leaned against the wall, her palms sweaty. Carl had knocked down a stone pillar, and the echo was still reverberating in the cave, giving one a headache.

The light came back on, a pale blue, illuminating four pale faces.

"If this keeps up, I'll start dreaming that I'm a night owl." Chen Hao rubbed his temples.

Nana glanced down at the data stream on the inside of her arm. "Energy fluctuations, affected by moisture. The next shutdown time is unpredictable."

"So, we'll have to get used to walking in the dark from now on?"

"It is recommended to maintain formation and keep the distance between each other no more than two meters."

"Sounds like military training," Chen Hao grinned. "Too bad there's no whistle."

They continued forward. The passageway branched out more and more, until five paths lay before them, each looking exactly the same. The rock walls gleamed with moisture, making them indistinguishable.

Chen Hao stared at the ground. The footprints had long been submerged in water. He looked up for the stone arrowheads he had left earlier and found that one of them had been moved.

"Who has touched this stone?" he asked.

No one spoke.

“It wasn’t me,” Carl shook his head.

“I didn’t go near,” Susan said.

Nana stood still, the light sweeping across the pile of rubble. "It's probably due to airflow vibrations causing the shift."

"This cave even has its own trail markers?" Chen Hao scoffed. "They're quite customer-oriented."

He walked along the left wall, drawing a line on the ground at each fork in the road. His fingernails scratched white marks on the stone, but by the third line, he noticed that there were already similar marks ahead.

“We’re doomed,” he said. “We’ve turned back.”

“Impossible.” Carl pulled out his notebook. “I’ve been keeping track of my steps. I’ve walked at least four hundred.”

“Then you should switch your business to selling maps,” Chen Hao said, pointing ahead. “Look at that rock, it has a V-shaped crack on it. I saw it ten minutes ago.”

Susan leaned closer and saw that it was true.

Nana pulled up the data model. "According to the formation pattern of karst caves, the main passage usually extends along underground water veins. We can find areas with dense dripping water to determine the direction."

“That’s right.” Chen Hao nodded. “The problem is—which drop of water doesn’t come from the sky?”

Water droplets kept falling from above, creating sounds everywhere. Some were crisp, some were muffled, and it was impossible to tell where there were more or less.

Carl tried collecting water in the bottles for a while, and found that the three bottles contained roughly the same amount of water.

“This cave is deliberately trying to keep us from getting out.” He stuffed the bottle back into his bag.

After walking a while longer, the lights went out again. This time it lasted for almost two minutes.

In the darkness, no one spoke. The sound of breathing became the only clue.

Chen Hao slowly squatted down and reached out to touch the ground. A cool sensation rose from his palm. He suddenly stopped.

"etc."

He took off one shoe and stuck his foot out.

"What are you doing?" Susan asked.

"Don't make a sound."

He closed his eyes, and a slight breeze brushed past his ankles.

Ten seconds later, the light returned.

"That entrance on the southeast side," Chen Hao pointed to one of the passages, "there's wind there."

"How did you know?" Carl frowned.

"My feet are cold."

"What?"

"A draft. Only the path leading outside has air circulation. Dead ends are stuffy."

Nana immediately scanned the carbon dioxide concentration. The data showed that the value was low in the southeast exit area, indicating that gas exchange was indeed occurring.

“Confirmed,” she said. “There is an external ventilation path in that direction.”

"He's definitely channeling his inner genius." Chen Hao put on his shoes. "Let's go, let's try this road."

They changed formation, with Chen Hao leading the way, crawling along the wall. Every ten meters he would stop, breathe on his palms, and watch the direction the mist dissipated.

"The wind has weakened," he said.

"Adjust the angle," Nana prompted, "Take a 15-degree turn to the right."

They turned into a narrow side passage. The passage became increasingly lower, until finally they could only move forward by bending over. Carca hesitated twice, cursed a couple of times, but still forced his way through.

Susan followed closely behind, her hand always on the rock face. Her fingertips touched a dent, and she stopped to look.

“There are scratches here,” she said.

Everyone gathered around. The symbols were the same as before, but arranged more densely.

“It’s that same sign again,” Carl said. “Could it be 'Dead End'?”

“It doesn’t look like it.” Susan looked closely. “This set looks like arrows superimposed with wavy lines.”

"Is the wavy line water?" Chen Hao guessed.

“It could also be wind,” Nana said. “Some ancient civilizations used curves to represent airflow.”

"So that means—we're on the right track?" Chen Hao laughed. "Our ancestors are showing us the way."

They continued forward. The wind gradually picked up, making their clothes cling to their bodies. The air was no longer stuffy, and breathing it in felt refreshing.

When the lights went out for the fourth time, no one panicked.

"One!" Chen Hao shouted.

"Two!" the three shouted in unison.

"three!"

"Four!"

After the numbers were counted, the team had rejoined in a straight line.

When the light returned, they were standing before an extremely narrow crevice. It was less than half a meter wide and bottomless. The wind was blowing out from inside.

Chen Hao reached inside. The wind had noticeably picked up, carrying a hint of coolness.

“This wind…” he paused, “is alive.”

Nana stood behind him; the lighting module flickered a few times before stabilizing. Her system indicated that the space within fifteen meters ahead was open, likely an exit area.

But she didn't say anything.

Susan and Carl leaned against each other, too exhausted to speak. Their faces were covered in a mixture of mud and water, and their clothes were torn in several places by the rocks.

"Shall we go in?" Carl asked.

Chen Hao didn't answer. He stared at the crack, his hand slowly moving forward with the wind.

Deep within the crevice, a faint light reflected from somewhere on the rock ceiling. It wasn't Nana's light source; its color was paler, like natural light filtering in from the outside.

He swallowed hard.

"Get the rope ready," he said. "I'll go first."

He knelt on one knee, took off his backpack and handed it to her. Then he squeezed sideways into the crevice. His shoulder got stuck as soon as he entered.

"Have I gained weight?" he muttered.

“You ate two packs of compressed biscuits yesterday,” Carl reminded him.

"That's to replenish energy." Chen Hao rubbed hard and finally slid in.

It was slightly wider inside than outside, just wide enough to walk upright. He turned on the flashlight strap and shone the beam forward.

Fifteen meters away, the rock strata broke off, revealing an upward-sloping path. That's where the wind was blowing in.

He turned around and waved: "Come on! This road is passable!"

Susan was the first to catch up. She used both her hands and feet, climbing slowly but steadily. Carl followed closely behind, getting even more stuck than Chen Hao when trying to get into the gap.

"I should have known better than to have that extra meal." He squeezed in, holding his breath.

Nana was the last to enter. Her metal casing scraped against the rock wall with a hissing sound, but she moved swiftly and without hesitation.

The four people passed through the crevice one after another and stood on the gentle slope.

The wind picked up. It blew against my face, carrying a freshness I'd never experienced before.

Chen Hao looked up. At the end of the slope, a narrow crack appeared at the top of the rock. Grayish-white sunlight shone in, casting its light on the damp stones.

"We're almost there," he said.

He started walking uphill.

He slipped and fell to his knees. His palms braced against the stone surface, touching a rough, carved area.

He looked down.

It's that same symbol again.

But this last beam points diagonally upwards, directly towards the source of the light.

He looked up following the direction the finger pointed.

The light spots were dazzling.

He raised his hand to block some of the light.

A tiny speck of dust was slowly rising with the airflow along the edge of my finger.