The ruins are at the tip of a diluvial highland which gently slopes from east-to-west, and two kilometers from the present-day coast line of the Seto Inland Sea.

The site was discovered on June 24, 1962 by three students from Harima Junior High School who were interested in archaeology and who had heard that a large quantity of earthenware shards had been unearthed during the construction of a nearby railway line in 1923.

Exploring the area, they discovered shards of earthenware, ironware, whetstones, shells, octopus jars, and other artifacts.

A local teacher identified the earthenware as Yayoi pottery and a report was filed with the Harima Town Board of Education.

[2] The artifacts excavated from the Ōnaka site are on display at the Harima Town Folk Museum (播磨町郷土資料館, Harima-chō kyōdo shiryōkan).