"Master, it seems that the kingdom of playing cards is safe indeed." Natalie exclaimed after they reached the ground and said goodbye to the guard diamonds.

"You're right, and I hope they continue to be." William replied with a sigh of resignation.

They passed Checkpoint 35, which gave them a charge of 2.5% of the energy.

The road underfoot winded its way like a snake, undulating because of the terrain. The needle of the compass was spinning. With the faint red light of the manjusaka, they could see clearly up to three meters ahead of them, so they wouldn't have themselves tripped over.

On the right side of the path, a glass tunnel appeared leading to the underground. The compass told them to go in, but when they saw the darkness, they were all too afraid. However, what made them more frightened was that they saw blood at the entrance and heard what sounded like crows screaming down the passage.

"What... What can be inside?" Natalie's tires were shaking violently. "Gosh, I feel like we're going to have to be invisible." Said Photon nervously.

"That's right. Upgrade the alert level!" William said. Then the invisible sphere fighters flew into the glass tunnel.

They would have turned on the lights of the sphere fighters, but the blood at the entrance warned them against doing so, so they had to fly extremely slowly. Natalie flew behind Photon so she could see the light from the exhaust port of Photon's sphere fighter. The light caught a few lines of words on the ground, so she told the others to stop.

"Here's a hint." She said. Rebecca leaned over and read,

Do you like to play hide and seek, my friends? I guess your answer is surely yes. But if I told you we were going to play hide and seek together with a very dangerous object, would you still say that?

Oh, fine. In any case, you must play hide and seek with it, and you must not be seen by it until you reach the end of the tunnel. When it appear, you will see a blood-red light. Hide yourselves toward both sides at once! Or you will...

"Good heavens!" Said Rebecca, trembling as she finished reading these words.

"What would that object be?" Natalie asked, puzzled.

"Nobody knows. But for the sake of figuring out its identity, let's go forward!" William led the others on through the terrible darkness.

They reduced the power of the sphere fighters as much as possible to comply with the hints of blood. The darkness lay like a hard, heavy stone upon them. It was extremely quiet, even somewhat suffocating. William could clearly hear his own heart racing and felt him gasping for breath. His forehead was wet with sweat, but he had no time to wipe it. He had a more important mission -- to avoid being found by "the dangerous object".

"I suggest we practice quick hiding first." Photon said in the radio. Before they could react, all the sphere fighters retreated to either side of the tunnel.

"Oh? Do the sphere fighters operate themselves?" Asked Rebecca, surprised and delighted. Her sphere fighter showed, "Yes. That's why our AI systems are so powerful."

Suddenly, a red light lit up, and the sphere fighters that had just flown out quickly retreated. The air froze in an instant.

The party stared at the blood-red light outside. Their muscles stiffened, their nerves were tense, and their blood pressure was skyrocketing.

The light made a strange sound,

"Hiss... Hiss..."

The sound echoed endlessly through the tunnel. William turned pale with fear. "I must figure it out." He deliberately controlled the frequency of his blinking so that he could see "the dangerous object". Unfortunately, he received nothing but a pungent smell and that sound.

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Now he was in a panic. "What on earth is that? Why can't I see it?" He thought over and over, "Can it be invisible too?"

He gasped at the thought.

At last, after an unknown amount of time, the red light disappeared. Sphere fighters flew into the tunnel. "My God, my heart is about to jump out of my chest!" Rebecca gave the propeller a few nervous turns.

"But did my master wonder why we couldn't see it?" Aria asked. "Yeah, I've been thinking about it." William looked nervous, "I now think it's able to be invisible, just like us."

"Aargh..." Ranran's red lips trembled. Her heart sank.

"Does that mean we can't deactivate being invisible if we're still in the tunnel?" Whispered Photon.

"I have no idea, but I suggest not." William replied, flying on forward.

As they went on, the party became more and more convinced of William's hypothesis. "If this is true," Thought Photon, "then it is terrible. For if the 'dangerous object' had as advanced a technology as ours, then Master's suspicions about that unknown weapon might be true, too..."

The sphere fighters flew slowly. The red light shone again, and the compass reminded them. As before, the sphere fighters hid quickly. A note fell by William's side, but he did not want to pick it up now.

The hiss came closer and closer to them. Ranran's face was full of fear. According to the sound and smell, she thought the object was approaching them. But a moment later it walked away again and disappeared.

Aria's sphere fighter quickly picked up the note. It read,

When God closes a door for you, he opens a window for you.

"What does this mean?" She was at a loss.

"Close a door, open a window... Well, I think they mean something else." Whispered Photon.

William supported his idea. "Did you ever find yourself hearing and smelling a lot better when you were hiding?" He asked Ranran.

Ranran did her best to recall. "Yes, I seem to have discovered that." She replied. "Gentlemen, I sort of know what that means." Rebecca said, based on the words of them two, "I think it's a reminder that since we can't see the 'dangerous object', we need to use our other senses."

They all agreed that her explanation was reasonable.

With the flames of the sphere fighters in front of her, Natalie could see strangely-shaped stones on both sides and overhead. Some of them were as sharp as claws, while others were as round as traps. Skeletons were placed on some of the stones, and it was a good thing that the party didn't pay too much attention to the skeletons -- otherwise they would have noticed that those skeletons were always facing them with a faint red glow in their eyes.

Natalie saw words on the ground that indicated that they were almost at the exit. "Hold on a little longer, everyone!" She cheered up the others.

The party could hear screams behind them. Though it wasn't loud, yet it stuck like a sharp needle into their bodies, injecting the hormones of fear into their bloodstream. William covered his ears in pain, but to his surprise the sound was louder. It seemed to know that William did not want to hear it, so it was further intensified.

"Oh, God, let me go... This is too terrible!" He said to himself. Soon a note appeared at his feet,

Let you go? No way! Don't expect it to weaken by one point before you go out!

Fear and anger were interwoven in William's body. He desperately wanted to get out of this horrible tunnel quickly, but they still had to avoid "the dangerous object". "How many more places like this do we have to go before we get back?" The question echoed in his mind. It kept hammering at his skull like a heavy hammer. He looked in pain and pressed his head hard.

"William!" There was Ranran's voice on the radio. At last the terrible question vanished, and William came to his senses. "Oh... What did I just do?" Said William, touching his sweaty forehead.

"Master, you must have thought of something terrible. Relax, and we'll be out soon." Natalie comforted him.

He shook his head violently to sober himself up. Strangely-shaped stones still lay everywhere, and skeletons stared at them. The invisible cage lay heavier on their shoulders. But William could see the ground ahead rising. Moments later, faint starlight shone on the sphere fighters' shells.

He breathed a sigh of relief, "We're finally out!"

But this was not the end. His hand slipped and touched a button on the console. The sphere fighter was instructed to open the magazine below and drop a small empty container. Unfortunately, it just touched a huge button. Then a blood-red glow lit up the tunnel, and screams rang through the air. The hiss came from the rear, approaching them quickly.

They were stunned for five seconds. Then William shouted at the top of his voice,

"RUN!"

The sphere fighters shot out bright flames and disappeared into the night.

William was frightened to death. He gasped, listened to his heart beating and felt that the sight turned red -- the result of the rising blood pressure.

"How stupid of me!" He shouted, blaming himself, "I nearly killed everyone..."

Photon comforted him, "All men can be nervous, Master."

"I hope that 'dangerous object' will never be seen again!" He said hastily, mopping a torrent of sweat from his forehead.

"No, Master." Said Rebecca, "It won't come to us again."

But no one could prove Rebecca right. If Rebecca wasn't telling the truth, then where would they meet it again?