Maruyama Kofun, located in the town of Asuka in southern Nara prefecture, is the sixth largest kofun, or ancient Japanese burial mound, in all of Japan. Maruyama Kofun is classified as a zenpokoenfun, or giant keyhole shaped tomb, which was the predominant style of monumental burial mound in Japan for roughly three centuries, from about AD 300 to AD 600.
Maruyama Kofun measures 318 meters in length, and looks like a massive wooded hill, but in truth was entirely man-made. The stone lined crypt inside the mound measures 26 meters in length, making it the longest of its kind yet discovered (most of the largest kofun have never been excavated). Two stone coffins were found inside, and were at one time believed, based on local tradition, to be those of Emperor Temmu and Empress Jito, who reigned at the end of the 7th century, but this was later disproved when the tomb was decisively dated to the 6th century, a century too early. At present the identities of the tomb's occupants remain unknown.
Maruyama is usually not listed in English guidebooks, but it is well worth a visit to get a sense of the truly massive scale of the keyhole-style kofun. It is also one of the largest kofun that they actually let you walk on. To get there, take the Kintetsu Railway to Okadera station. Maruyama Kofun is just a block or so to the north.