William piloted the sphere fighter. The computer map showed that they were approaching the next checkpoint soon. Sure enough, they made it through Checkpoint 38, which gave them a charge of 2% to their energy core.

"We need to land," Ranran said over the radio. Then the white spherical shadows in the night sky dropped their height and settled themselves firmly on the ground.

Aria looked around. It couldn't have been more ordinary: there were strangely-shaped stones everywhere. Some of the larger stones were quietly sleeping under the night sky, draped in the quilt of starlight. On either side of the road were grasslands where low fluorescent grass grew. They were crystal clear, almost like jewels, and emitted a light similar to the instant flowers.

"Wow, that's beautiful." Ranran could not help but sigh with admiration.

But they soon noticed something unusual. "Look!" Rebecca tapped the propeller against a stone, "Look there! The grass forms words!"

The others came. Sure enough, the fluorescent grass stood out in huge words.

Help!

"Oh? What does this mean?" Asked William, puzzled. Then, by the light of the fluorescent grass, he saw a piece of paper sticking out from under the stone.

"There's writing on it, Master!" Photon said, "You take it out and have a look."

William pulled out the note. Sure enough, there were words on it,

Save this imprisoned scorpion! Help me escape, and you'll have the treasures!

"Treasures? What treasures?" Ranran was at a loss. The party was more confused. However, they still followed the directions of the note until they came to a wide cave.

There was endless darkness in the cave, so they could only rely on the red light of the compass to see their way. The heavy air pressed down on the party like a heavy stone, leaving them breathless. William felt the invisible hand approaching him silently again, for his breathing became more rapid and difficult.

"Are you all right?" Ranran pulled hard at William, even though she was uncomfortable, too.

"It's okay, don't worry about me." William gasped out the words.

Suddenly, the air was no longer heavy. Then, a button appeared at their feet in front of them. Curious, they approached to find out. Natalie found the brief note next to the button,

This button is portable. Press on different parts of it to get gravity in different directions. Use this function to walk through the tunnel ahead, and you will be able to save the scorpion.

"Oh! Gravity in different directions!" Rebecca laughed, "That sounds good!"

"I think you'll like it." William picked up the button and led the party on through the dark cave.

They came to the tunnel very soon. Rebecca noticed something special about it: the top of the tunnel was a flat surface, but the bottom was covered with what appeared to be detectors. Photon knew from the handwriting beside that those were pressure sensors, and when they detected pressure, they would trigger the laser weapon system.

"So it's time to use this, my friends." William said, "We're about to be able to experience what walking upside down will be like. However, in the process of changing the direction of gravity, you need to adjust your posture. In this way, when we reach the top of the tunnel, you will be able to walk 'normally' as you do now."

He pressed the top of the button. The party left the ground at once and floated slowly upward. Ranran danced happily, her face brimming with a huge smile. "Wow, I'm flying! Ha ha ha......" Aria smiled. "Now you can be a flying locomotive." Ranran added.

They drew graceful arcs in the air and landed smoothly on the top of the tunnel. Looking back, Natalie saw that the road had turned upside down. "I can't believe we've been walking upside down!" She said. Cheerful laughter echoed through the tunnel.

"Now, you can't do anything about us, can you, poor detectors?" Said William, moving on.

The light of compass cut through the barrier of darkness and made them realize that they would soon need to change the direction of gravity again. This time, they needed horizontal rather than vertical gravity.

Natalie cried, "Oh! I thought we had all the fun. Looks like it's not over yet!"

William pressed the button on the right. They floated up again and landed on the right side of the tunnel. "We even learned to walk sideways!" Laughed Photon. He made funny faces at the detectors "above" as he advanced. Rebecca thought he would be furious if there was someone behind the detectors.

The tunnel ahead became more complex, but more interesting too: the detectors were always distributed symmetrically. "Are you ready for a dance of mirrors?" William laughed.

"What?" Ranran asked, "What does that mean?"

"Dancing in the air while switching gravity direction symmetrically, of course!" William replied.

They had experienced upward and horizontal gravity, but that was only the beginning of the joy, while the real joy would not come until now. The party was floating around like balloons, perfectly dodging all the detectors.

"Trying to puzzle us? Dream on!" Laughed Photon contemptuously.

More exciting was yet to come. The detectors had a rotating distribution starting from the top of the tunnel, and they were distributed in different locations. "Well," Said Photon, looking somewhat worriedly at the detectors' spiral, "that doesn't look so easy. Master, are you sure we can get through? Let's be invisible!"

"Wake up, Photon. Being invisible won't work because we can still transmit pressure to the detectors." William replied, "Well, as difficult as it looks, it's nothing. Now that we know where the detectors are distributed, we can always be in the opposite position to them! In other words, the detectors' distribution trajectory and our path will be two different helixes whose points on each circular cross section will have a certain diameter."

The others were suddenly enlightened. "Master has a point." Aria said, "But if you do it that way, you'll be very tired. You need to calculate the right direction of gravity at all times and coordinate our speed. And if we're not on the same cross section all the time, your job is going to be much more complicated."

"Yes, it's a big job, Master." Said Rebecca, "But you may rest assured that we will obey your orders."

"Good." William looked at the tunnel in front of them by the light of the compass, saying, "Obviously, it's impossible to put everyone on the same section. As far as I can see, we can narrow the spacing so that we can get through. But I have a cleverer idea."

"Oh?" The others looked at him in bewilderment.

"Now that we have such beautiful helixes, we must make use of its special properties. We can easily see that helixes are periodic, so we can divide ourselves into parts. If every part is in the same phase and moving at the same speed, we should be able to get through. What do you think?"

"That's a good idea!" Photon said.

"Yet the challenge may just be maintaining the same speed." Said Rebecca.

William replied, "I walk at a speed of about 5km/h and I believe everyone can do it. You guys can use the speedometers, while I can just relax."

Having obtained the unanimous agreement of all, they began to move forward. Ranran focused on controlling her stride length and cadence, while the rest, including the sphere fighters, kept an eye on their speedometers. Finally, after about a minute, they passed "the helixes".

"Yay!" Everyone cheered loudly. They had gone through all the tunnels. Now, in front of them was a huge glass case with a scorpion inside. Its body was covered with various tubes, where blood was flowing. The scorpion's expression was very painful, and it was struggling desperately. "Help me!" Cried the scorpion, very excited and exhausted, when it saw the party in front of it.

"Poor him!" Said Rebecca miserably, "He will probably bleed to death soon! We must save him!"

"How can we save you?" William asked through the thick glass.

"Look at the instructions; I'm losing blood... I can't tell you..." Said the scorpion weakly.

William had to consult the instructions on the ground. The light of the compass shone upon these words:

Use the items in the toolbox next to you and break the glass to save the scorpion. The natural frequency of this glass is already known.

"Natural frequency?" Thinking, William opened the toolbox and saw a trumpet. He banged his head and cried excitedly,

"I know!"

So he turned on the power switch of the trumpet, put it to his mouth, and first gently shouted, "Roar!" He was surprised to see that what the frequency indicator read was close to the natural frequency, as well as that the glass was already shaking slightly, though it was barely noticeable.

Then William took a deep breath and shouted with all his strength, "Roar!" The sound was deafening, rising into the dark night sky. The glass shook violently and broke into pieces. Tiny fragments flew in all directions like fireworks. The blood in the scorpion's tubes stopped flowing immediately.

The scorpion was saved.