The first sleep after being woken up, Liu Xuan tossed and turned in bed, unable to sleep, this new era is developing too fast, a little unimaginably fast. Liu Xuan didn't think about today's pattern a hundred years ago, but he didn't expect that technology would develop so fast.

A hundred years of time has given Liu Xuan the feeling that he has spanned a thousand years. The potential of mankind has been fully exposed, and compared with the beginning of the 21st century, the development of science and technology is even faster, which is simply a "big bang of science and technology".

In the early morning, when the sun sowed again, Liu Xuan and Li Ping had already gotten up early, made an appointment with the driver, and went to the entrance of the underground city.

"Yesterday, I applied overnight to increase access to the dungeon to the chip in your body." After Li Ping pulled Liu Xuan into the flying car, he said simply.

Liu Xuan nodded lightly, retracted his gaze looking out of the window, and felt infinite emotion in his heart, he didn't expect that it would be a hundred years later to go to the dungeon again.

Dozens of minutes later, the flying car came to a steady stop next to a light blue pillar of light, and there were two signs next to the pillar of light, one that read "The Birthplace of Aviation" and the other that read "Danger".

Liu Xuan was stunned when he saw these two brands, what does this mean, dangerous? Liu Xuan moved his gaze to Li Ping and asked the meaning of the two brands with his eyes.

"This is the emerging beam elevator that combines transport and attack. When you have access to the underground city, the beam energy is reduced, and you can be sent to the underground city steadily, similar to the function of an ordinary elevator. If you don't have permission to get close to the beam, you will be turned to ash by the beam of light within a ten-thousandth of a second. Li Ping introduced emerging high-tech products to Liu Xuan, and also indirectly explained why he applied for permission to Liu Xuan overnight.

Li Ping pulled Liu Xuan into the pillar of light first, the pillar of light turned into a transparent color, and the bodies of the two people began to slowly descend, heading towards the depths of the ground.

"What are these things?" Liu Xuan asked, pointing to the small hole on the side of the pillar of light.

"Attack the system. This elevator is still a bit but it seems, under the protection of a person with authority, there is still a chance to bring outsiders into the underground city, in order to prevent this kind of thing from happening, and an intelligent attack system has been added to the edge of the pillar of light, which can kill the intruder on the premise of protecting scientific researchers. Under Li Ping's explanation, Liu Xuan's scalp tingled, which meant that the intruder had only a dead end.

"Is this underground city so important?" At this time, the beam elevator has been running into the pseudo-city above the real underground city, this city was built to protect the real underground city, Liu Xuan was not inhabited when he came, at this time, Liu Xuan could see the people in the city.

"It's very important that some advanced scientific research should be carried out in underground cities. The uninhabited city above has also begun to arrange people to live, they are all frozen, and they are not adapted to the new era before entering the underground city life, but they also don't know that there is a more important city under their city. Obviously, Li Ping woke up early and did a lot of research work in the past few months.

Liu Xuan nodded and chuckled.

"Also, you were thawed in North America at the time, and everything was normal before you shipped back to Asia." Li Ping said to Liu Xuan with a smile.

Liu Xuan didn't care about any of this, he didn't know anyway.

Finally, the beam elevator came to the end, and the underground city that Liu Xuan was familiar with appeared in front of him, except that the house was a little dilapidated, it was really the same as Li Ping said, the underground city had not changed at all, and even the number of buildings in the city had not increased or decreased.

If Liu Xuan hadn't seen the changes on the ground with his own eyes, Liu Xuan wouldn't have believed that a hundred years had passed in a flash.

According to the familiar route, Liu Xuan returned to his small room again, to his surprise, the small room was still clean and bright, without a trace of dust, on his bedside table, his family photo was still there. His mother and father are already a figure from a hundred years ago, although Liu Xuan still believes that his father is still alive.

"I expected you to come back, so I cleaned it, and the photos were kept for you back then, and now they are back to their original owners." Seeing Liu Xuan's excited face, Li Ping explained to Liu Xuan.

"Thank you." After Liu Xuan finished speaking, he gave Li Ping a big hug, which caught the calm Li Ping off guard, but fortunately Liu Xuan just hugged him politely and did not make too many movements.

"Let's take a look at the lab again." Li Ping blushed slightly and took the lead in walking out of Liu Xuan's room.

Liu Xuan had no objection, and the two of them walked through the city as if they were a hundred years ago, walking towards the research room that they were already familiar with.

On the road, pedestrians coming and going, wearing the most primitive clothes and using the most primitive tools, Liu Xuan felt that everything was like a dream, whether it was his contact with the lifeforms of the seventh-level civilization or the high-tech life on the ground.

"Let's go see Professor Ma first." Liu Xuan changed his doctrine and walked towards the edge of the city, Li Ping had no objection, for her, it was the same wherever they went first, and her time had passed.

"Can you tell me about the development of aerospace in the past 100 years?" Liu Xuan asked at Li Ping.

"With the help of your nuclear fission engine, people have successfully developed a nuclear fusion engine, and with these two powerful engines, it is not a dream to travel around the solar system at will. The wormhole was still there, and we had launched a special wormhole satellite to monitor the changes in the wormhole at all times, and the only change was that there was no change. However, human scientific research has also caused the depletion of the earth's resources, in order to collect space resources, we built interstellar warships, one is to resist future space invasions, and the other is to bring interstellar materials back to the earth, the most exaggerated one, they actually pulled an asteroid with a very high gold content back to the earth, according to records, the landing point was set in the Pacific Ocean, when the worst tsunami and financial crisis in history was formed, and the price of gold plummeted. Li Ping shook her head, helplessly saying that these people are really crazy.

"Aero engine research has reached a bottleneck again?" Liu Xuan just heard the question he cared about.

"Yes, there are two research directions, and they are divided into two groups, and they are dedicated to their own engines. One faction believes that more abundant energy should be found, so it advocates the theory that all materials can be fused, and wants to completely burst out the energy of a stone and use it for interstellar navigation, so that there is no need to worry about running out of fuel. The other faction is more radical, and they are committed to the study of light-speed ships. So far, neither faction has succeeded. ”

Liu Xuan nodded, as if he suddenly thought of something, and asked abruptly, "Have you ever heard of the theory of the constant mass of the planet?" ”