Because the moon has long been "tidally locked" by the earth, this will cause the cycle of the moon's rotation to be just equal to the period of its revolution around the earth, so that the moon will always face the earth on the same side, and human beings on the earth have not been able to see the other side of the moon, so for a long time in the past, human beings can be said to know nothing about the far side of the moon.
This changed until 1959, when the Soviet Union launched a lunar rover called "Luna-3", which was the first to take pictures of the far side of the moon, although these photos were of lower resolution and covered only about 70% of the far side of the moon, but it also marked the official start of human exploration and research on the far side of the moon.
(↑ The far side of the moon taken by "Luna 3", which is also the first real photo of the far side of the moon seen by humans)
Today, the far side of the Moon is no longer a "blind spot" for humans, and through a series of lunar exploration missions, we have obtained a lot of information about the far side of the Moon, among which there are of course some eye-catching discoveries.
Scientists, for example, have discovered that there is a large foreign object beneath the center of the South Pole-Aitken basin on the far side of the Moon, which causes a significant anomaly in the gravitational field in its area.
Seeing this, some people may ask, how do scientists know what the moon looks like underground? In fact, the principle is not complicated.
To put it simply, when the probe is flying around the moon, it will be affected by the moon's gravitational field, which in turn depends on the mass distribution of the moon, the probe can sense the subtle changes in the moon's gravitational field through microwave measurements, and transmit the data back to the earth, scientists can build a model of the moon's gravity field by analyzing these data, and then infer the mass distribution inside the moon.
Studies have shown that this huge foreign object found on the far side of the moon resembles an ellipse in shape as a whole, and extends about 300 kilometers downward, with a cross-section of about 5 to 60,000 square kilometers, and its mass density is much higher than that of the surrounding area, which indicates that it should be made of metallic material, and according to scientists' estimates, its mass can be as high as 2180 trillion tons (2.18 x 10^18 kg).
So, what exactly is it? For this question, people who believe that the imagination is rich have already thought of the legendary extraterrestrial civilizations, because in some legends, extraterrestrial civilizations have been observing humans in a way that is difficult for humans to detect, and if this is the case, then they may build their bases underground on the far side of the moon, after all, it is difficult for humans on Earth to discover.
Of course, the option of "extraterrestrial civilization" will not be adopted by the scientific community without conclusive evidence, and in fact, scientists have also proposed two reasonable explanations for this huge foreign object found on the far side of the moon from the perspective of natural formation.
The first theory is that the Moon was caused by a massive impact event that occurred about 4.5 billion years ago, when the Earth was still a "young" protoplanet, and it was struck by another protoplanet the size of Mars, known as Theia, in which the Earth and part of Theia were ejected from orbit, forming a cloud of debris around the Earth, which then gradually gathered and eventually formed the Moon.
In this process, the primordial moon will be in a high-temperature molten state as a whole due to the high temperature generated by the collision, and in the subsequent cooling process, due to the influence of the earth's gravity, the "gravity differentiation" of the primordial moon will be uneven, which will cause a part of the metal-rich magma mass to gather in a certain area of the shallow layer of the moon, and eventually form a huge foreign body.
Another explanation is that the giant foreign object found on the far side of the moon is supposed to be the remains of an ancient asteroid.
To put it simply, a long, long time ago, a giant asteroid hit the moon, because the angle of impact is relatively positive, and the planet is very dense, at least its core is composed of a large amount of metallic materials, so when the impact occurs, the main structure of the asteroid does not break or eject, but deeply crashes into the interior of the moon, and has since become a part of the moon.
As it stands, both explanations can only be said to be reasonable speculation, and what the actual situation is remains to be further explored. It is worth mentioning that the mainland's Chang'e-4 lander and Yutu-2 lunar rover are currently exploring the area, and we hope that in the coming days, they will help us unravel this mystery.