Iwayayama Kofun

The mound was long thought to be a square hōfun (方墳) tumulus with 45-meter sides. Modern archaeological techniques indicate that it was most likely square in the lower section with an octagonal upper section. Excavation showed that it was built in two stages, with sides of about 45 meters in length and a height of 12 meters, and that the mound was carefully tamped.

The burial chamber is a double-sided horizontal-entry type made entirely of granite, with elaborate cut stone processing applied to the inside. The walls are stacked in two layers, with one cut stone above and one below on the end wall, and two cut stones above and three below on the side walls, the upper layer of each wall tilting inward. The ceiling stone is a large monolith. The side walls of the passage are single-tiered in the back half and double-tiered in the front half. The chamber is 4.9 meters long, 2.7 meters wide and 3 meters high, and the passage is 12 meters long and 1.9 meters wide.

In the area from Nara Prefecture to southern Osaka Prefecture there are several kofun with the same layout and proportions, notably the burial chamber of the Eifuku-ji Kita Kofun in Osaka Prefecture (said to be the tomb of Prince Shōtoku).

In 1978 the western side of the mound, previously eroded, collapsed due to flooding. Environmental improvement work was then carried out and excavations could be undertaken. The burial was first reported to the academic community by the Englishman William Gowland in 1897. As the chamber has been open for centuries, no grave goods have been found.