Ishibutai Kofun

The kofun was built on a spur of a mountain that falls northeast to a small plateau. It consists of a platform, a gallery entryway, stone walls forming a burial chamber, two large stones forming its ceiling, and embankments on either side. In total 30 stones were used. The large granite megaliths come from Mount Tōnomine, approximately 3 kilometers from the site.

The inner chamber opens to the southwest and is 7.5 m long, 3.4 m wide and 4.8 m high. The total weight of the stones is estimated at 2,300 tons. The ceiling is formed by two megaliths, the northern weighing approximately 60 t and the larger southern one approximately 77 t. Small drainage channels run along the east, north and west of the tomb and feed into a shallow drainage channel in the entrance path.

The kofun was originally a large, flat hōfun (square) type, covered by a broad, flat earthen mound that eroded slowly and exposed the roof megaliths. It was built on a square platform measuring 50 m on each side and was surrounded by a moat estimated to be 12 m wide. The entrance path is 38 m long, and its stone ceiling no longer exists.

The kofun was first excavated in 1933 by the archaeologist Kōsaku Hamada. Work on the base and moat began in 1935, and excavation of the tomb continued until 1975. Because of its association with Soga no Umako, the tumulus has no imperial designation and has therefore been extensively excavated.

The excavation yielded no significant finds; funerary objects were probably lost to grave robbery soon after construction. Stone shards to the southeast are the remains of a tuff sarcophagus, and gilt and bronze implements and earthenware shards were found in the banks of the tomb approach. Seven small stone burial chambers once existed to the north and south of the structure; they are now believed to have belonged to earlier tombs on the site, levelled to provide building material.