The tumuli were collectively designated a National Historic Site in 2015 with the area under protection expanded in 2020.

[1] The Ōnohara Kofun cluster is located in the western part of Kagawa Prefecture, in the center of an alluvial fan formed by the Kushita River, overlooking the Seto Inland Sea.

The burial chamber opens in the south-southwest direction and is a single-room structure with a total length of 13.2 meters.

The Kakuzuka Kofun is a square tumulus located adjacent to Onohara Central Park.

Inside, fragments of Sue ware pottery were found, which were dated to around the first half of the 7th century The Iwakurazuka Kofun is located in the precincts of Onohara Hachiman Shrine, close to the Wankarizuka Tumulus, and excavations revealed that it was built over a portion of the outer moat of that tumulus.

During excavation, late quantities of flat stones with single-character Buddhist votive inscriptions dating from the Edo Period were discovered.