Kofun, literally "old tombs", are the mysterious, massive earthen mound tombs of ancient Japanese rulers. Many of these tombs are so large that when seen from the ground, they look just like huge, tree-covered hills, but when seen from the air they have regular shapes and are clearly man-made. Most famous are the keyhole-shaped "zenpokoenfun," but kofun also appear as squares, diamonds, circles, two conjoined rectangles, and ovals.
The kofun give their name to the Kofun Period of Japanese history, when they were constructed. The largest kofun, Daisen Kofun at Sakai near Osaka, is the largest tomb in the entire world by volume and by length, even larger than the Great Pyramid at Giza or the tomb of Emperor Qin Shihuangdi near Xian, China.
What makes the kofun so mysterious and interesting is that we are still not really sure who built them, or exactly why. This is because they were built several hundred years before writing arrived in the Japanese isles from China, so we have no historical documents about them. While various legends exist attributing certain kofun as the tombs of certain legendary emperors and noblemen, there are *far* more kofun than emperors, and the kofun are scattered all over Japan and not just in central Japan where the emperors supposedly lived. This suggests that far from being the unified empire that legends suggest, Japan was actually much more splintered politically, with many distinct socio-political centers.
Another mysterious aspect of the kofun is that we are not really sure what was inside most of them. This is because in the Meiji Period (1868-1912), when archaeology was just getting started in Japan, the Japanese government gave most of the kofun over to the Imperial Household Agency, on the logic that they contained the ancestors of the Emperor, and since the Emperor was considered a living god, it was deemed a sacrilidge to dig up the graves of his ancestors. This ban on excavating the kofun continues to this day.
A few of the smaller kofun have been excavated, however, mostly because they were too small and not recognized as kofun until after the Meiji Period. Mostly these kofun contain adorable haniwa clay figurines, and what were apparently the standard treasures of the day - lots and lots of bronze mirrors, nonfunctional decorative bronze bells, and Chinese coins.
However, one particularly well preserved kofun, Takamatsuzuka near Asuka, was sealed so tightly against the elements that gorgeous colorful paintings were preserved on the rock walls of the inner chamber all the way until they were first discovered in 1972. These paintings depict people wearing surprisingly Korean-looking garments, which, along with the presence of similar massive earthen tombs in Korea during the same period, lends credence to the increasingly accepted theory that the ruling classes of premodern Japan were of Korean extraction.
The larger tombs however, may contain more magificent treasures. When an 1872 typhoon exposed a small burial chamber in the forward part of Daisen Kofun, archaeologists found ornamental swords made of gold and copper, silver-plated armor, fine glassware, iron pots, and clay dishes, which makes one wonder what wonders must lie in the main chamber! However, despite the fact that scholars have petitioned for decades to get the Imperial Household Agency to allow further investigation, these appeals have fallen on defiantly deaf ears.
A third mystery relating to the kofun is why they suddenly stopped being built. The earliest kofun begin appearing in Japan around AD 300 and rapidly caught on, leading to an explosion of kofun-building all over the islands from about 400 to 550. Given the amount of labor that must have gone into moving that much dirt around, it is a tribute to the power and influence these rulers must have held over their subjects.
However, by about the year 600, kofun-building had completely ceased, and the early legends to not provide any explanation. A reasonable theory however, is that as one particular dynasty in Central Japan (the progenitors of the current Imperial House) came to dominate over all of the others, the need to compete with nearby rulers to build the biggest and most magnificent kofun declined. It is also probable that the arrival of Buddhism and and cremation and the reform of the court along Chinese lines increasingly suggested to the Japanese that building massive tombs filled with supplies for a journey to the afterlife was a primitive practice which ought to be abandoned.