Even before World War II the Germans were already in contact with Extra-Terrestrials particularly their Secret Societies. The Mediums also known as Channelers in today's terminology were mostly women. These women mediums are required to have long hairs, the longer the hair the better the reception of messages they receive from off world contacts. Some of the early German technology especially the UFO disc type of crafts were inspired by the ETs.
In one of the messages they received led them to research the forgotten history of the Earth. Atlantis was one of the topics they were keen to looked into as they found out that not only German people but most European people descended from Atlantis. In their understanding the UFO technology as well as advance technology for free energy were already in existence in the world in the time of Atlantis. All they have to do is to get in touch with the people of Tibet who have records of the past as well as in possessions of safely guarded advance technology which they hid in caves and underground bases.
It was in this expedition to the land of Tibet that the German people learned that the Earth was surprisingly "Hollow". Apparently, the Tibetan people not only knew that the Earth was hollow but said that some of the Atlanteans took refuge inside the Earth and that underneath the land of Tibet is an inner Earth City. The Monks of Tibet showed the Germans that there are two major openings to the Hollow Earth one in the North Pole and one in the South Pole. The Germans decided to use the South Polar opening so that it will not make much noise in Europe, so that no one would know much about it.
Several submarines were sent to the South Pole looking for the Polar Opening under the ice of Antarctica. Finally, they succeeded in finding it. Immediately plans were made about colonizing that unknown part of the world. Maps were made in order for ships and submarines can easily locate the opening to the Hollow Earth on the South Pole.
Fast forward to the days during World War II, when Adolf Hitler felt like their German Army are losing the war, Hitler decided to assign the German fleet of Submarines known as the "UBoats" to prepare and transfer to the land inside the Earth in which they call today as "Rainbow City". As more and more German towns were being occupied by the Allies on the East side of Berlin where the Russian armies were closing in. And at the same time on the West side of Berlin the American troops. Hitler and some of his comrades made a dummy or diversion to make it seem that they already passed away. But they escaped using underground pathways and then boarded the submarines.
Unknown to the world, Hitler along with his superiors, officials and soldiers migrated to South America. I will not mention which South American nation they went to. Hitler have to live and accept the fact that they lost the war in Europe. They then turn their attention to building the 'Rainbow City' and explore the Hollow Earth. They brought with them South American women to this new city to be their wives. The fleet of Submarines that were assigned to this expedition are the "U-Boats".
As German people were being transferred to South America, they learned that they can actually create an opening to the Hollow Earth right there in South America in addition to the South Polar entry. In one of the mountains they made a hole which leads to the Hollow Earth. Several years later Adolf Hitler died in South America.
In the year 1943, the Germans in South America were informed that indeed the Earth is hollow and that they are going there. But some of them were assigned to stay on the surface world. One of the German Submarine code named "U-209" was one of the last one to successfully enter Hollow Earth. The Commander of the submarine was Heinrich Brodda. One of the crew of this submarine named Karl Unger sent a confirmation letter to his German friend in South America that they made it inside the Earth through the South Polar entry.
The Germans soon found out that there is a much older Civilizations that exist inside the Hollow Earth. And that they were not allowed full entry inside confirming what was said by the people of Tibet. Instead, they were allowed to stay to a large land that exist just outside the Hollow Earth. And this is the place they call "Rainbow City". The location of this city is just several hundred kilometers from the South Polar entry.
The Civilization that exist inside the Hollow Earth is highly advance that even the latest technology on the surface world pale in comparison to what they have there.
The confirmation letter of one of the crew of the U Boat -209 named Karl Unger proved that indeed the Earth is Hollow! And if the Earth is hollow, then it means all the planets and even the moon is also hollow; having an interior world inside where civilizations actually exist.