Daisen Kofun, located in Sakai city just south of Osaka, is the largest kofun, or ancient burial mound, in Japan, and one of the largest tombs of any kind in the world. Daisen is a massive, heavily wooded, keyhole shaped mountain in the midst of a very modern metropolis. The scale of the thing truly boggles the mind - the whole mound is 486 meters in length, and at its widest in the round part of the "keyhole," is 286 meters in width, while the highest point towers 40 meters above the ground. By comparison, the Great Pyramid of Giza is 230 meters along one side of the base. And if the mound itself were not big enough, it is surrounded by three massive moats, bringing the total dimensions of the site to 793 meters by 656 meters. Nobody is allowed onto the site, but takes more than half an hour to walk around the edge of whole thing.
Daisen Kofun's keyhole shape classifies it as a zenpokoenfun (literally, "long front, round back tomb"), the style of monumental tomb that predominated during the appropriately-named Kofun Era in Japanese History (ca. AD 300-650). Little is known for sure about Daisen, but it is believed to have been constructed in the early 5th century, and according to popular tradition, is the resting place of Nintoku, the mythical 16th emperor of Japan. Daisen Kofun is part of a large grouping of burial mounds known as the Mozu Kofun Group, and the tomb is surrounded by 12 small tombs held to belong to Emperor Nintoku's attendants. Nearby is the nearly-as-large Misanzai Kofun (third largest in Japan), traditionally held to be the tomb of Emperor Richu.
Daisen Kofun has never been excavated, because as the purported tomb of an emperor it is considered sacred ground, but an 1872 typhoon exposed a small burial chamber in the lower, front part of the mound, in which was found a stone coffin as well as ornamental swords made of gold and copper, fine armor, glassware, iron pots, and clay dishes. At other times, clay sculptures of horses and people, known as haniwa figures, have surfaced on or near the mound, and the Boston Museum of Fine Arts exhibits armor and swords which are said to have been taken from the mound, although this has never been definitively confirmed.
It is baffling why the Japanese do not promote this amazing site more for its tourism potential. Daisen Kofun is not even mentioned in major travel guides such as Lonely Planet, but is truly one of the most amazing man-made constructions ever undertaken, and is well worth the 20 minute train ride from Osaka. To get there take the JR Hanwa Line to Mozu. The mound is about five minutes by foot and English signage is plentiful.