Kofun is a term related to one specific segment of Japanese cultural heritage. Kofun is the name of megalithic tombs, originating from the period between the third and the seventh century AD.
The period between the middle of the third century and early-middle sixth century is named after these tombs. Hillocks of many of these tombs have a very specific shape and they look like a keyhole.
The design of the tombs and their technical characteristics indicate that an ancient Japanese master was very skillful, but these structures rise much more intriguing question. Are they, in any way, connected to some extraterrestrial world? It looks like they might be.
As a matter of fact, NASA has noticed that a structure on Mars incredibly resembles those tombs in Japan.
NASA says that the structure on Mars could have appeared after the displacement in the crust of Mars. Erosive processes on Mars have been happening for millions of years, and they caused forming of all those neat lines. While this explanation is good and reasonable, the fact that there are no many unique extraterrestrial shapes so similar to shapes on Earth, still tickles the imagination.
Is it acceptable to think that ancient civilizations maintained contact with some extraterrestrial race? Are those interplanetary similarities a proof of interplanetary communication that once existed? The answer is still unknown.