The two most prominent tombs in the cluster are keyhole-shaped kofun -- that distinctive form, visible only from above, where a square platform joins a circular mound. The Hisagezuka Kofun stretches 101.4 meters in total length, making it one of the largest burial mounds in Ibaraki Prefecture. Its posterior circular portion alone measures 65 meters in diameter, and numerous haniwa -- fired clay figures of soldiers, horses, and houses placed around the mound's perimeter -- have been recovered from its slopes. These keyhole tombs were the exclusive burial form of Japan's ruling elite during the Kofun period. Their presence here, on a hill overlooking the ocean rather than in the traditional power centers of the Kinai region, speaks to the wealth and authority of whoever governed this territory between the mid-3rd and late 4th centuries.

The Kurumazuka Kofun takes a different form entirely. At 88 meters in diameter, it is one of the largest circular-type kofun anywhere in Japan -- a massive round mound without the squared-off forward platform of its keyhole neighbors. Circular kofun are generally considered slightly lower in the ritual hierarchy than keyhole types, but Kurumazuka's sheer size challenges easy classification. Construction techniques and grave goods date the cluster's tumuli from the mid-3rd century through the late 4th century, spanning a period when the Yamato polity in western Japan was consolidating its influence across the archipelago. The site also includes two rectangular double-joined kofun and two mounds of indeterminate shape, though many more tumuli once stood here before urban development and land clearing consumed them.

The kofun did not stand in isolation. The surrounding Higekama Ipponmatsu site preserves traces of a large-scale settlement that stretched continuously from the preceding Yayoi period into the Kofun era, suggesting that the people who built these tombs lived right beside them for generations. This necropolis was not a remote burial ground but the ceremonial heart of a thriving community. The site's commanding position -- a hilltop overlooking both the Pacific Ocean and the mouth of the Naka River -- strongly suggests that these local rulers drew their power from maritime trade and river navigation. The nearby Oarai Isozaki Shrine, one of the area's oldest Shinto shrines, is dedicated to the Nakakunizo clan, whose name echoes the Naka River itself and may represent a direct cultural link to the people entombed in these mounds.

Walking the site today, just a 12-minute stroll from Oarai Station, the mounds have settled into the suburban landscape of a quiet coastal town. Grass covers the earthwork contours. Trees shade the slopes where haniwa once stood in ceremonial rows. The 4,100-plus artifacts recovered from excavation -- the mirrors, the weapons, the curved jade beads that were already ancient symbols of power when these kings were buried -- now reside in museum collections. But the mounds themselves remain, designated and protected, a reminder that eastern Japan had its own centers of authority long before Edo or Tokyo existed. From above, the keyhole outlines emerge from the greenery like signatures written in earth, marking where rulers of a forgotten Pacific coast kingdom chose to spend eternity.