Speaking of aliens, everyone has a lot to talk about. When I was a kid, some adults told me that they had seen aliens and their plate-shaped aircrafts. In fact, none of them actually exist. Humans have a very complicated feeling when they face the deep space. They hope that humans are the only kind of creatures in the universe, meanwhile they feel sad to be unable to easily leave Earth. Sometimes the blue and thriving world feels like a cage for humans. In that case, people began to worry about being attacked by some other creatures from other planets. Aliens are usually depicted as evil and having advanced technology, like what we can see in Star Trek. That kind of fear existed throughout the human history. Until today, we can travel to the nearest planets, but we are still confused about whether aliens exist or not. Finally, we realize that we actually know very little about the universe. Besides the aliens from outer space, we even believe that there are people living inside the earth. Do you believe it? If not, do you have some evidence to prove it?