The tumulus sits on a low rise near the center of modern-day Ota, in eastern Gunma Prefecture. Its keyhole shape -- called zenpokoen-fun in Japanese -- is the hallmark of elite Kofun-period burial, reserved for rulers of significant power. The circular rear portion rises 16.5 meters high with a diameter of 120 meters, while the rectangular front section stretches 116 meters wide and stands 12 meters tall. Both sections are built in three carefully engineered tiers. The surface was originally armored in fukiishi, a layer of river stones that prevented erosion and gave the mound a striking, pale appearance against the green landscape. An inner moat, still intact, ranges from 24 to 36 meters wide, and a narrower outer moat once completed the monumental enclosure. A railway line now crosses the outer moat, and a prefectural road skirts the rear dome, but the core structure has survived remarkably well.

Excavations have recovered both cylindrical haniwa and figurine haniwa shaped like people and houses from the mound's surface. These fired-clay figures once stood in rows along the tiers, serving as ritual guardians and markers of status. Their design places the kofun's construction firmly in the early-to-mid 5th century. The tomb itself was robbed in antiquity, and the burial chamber's original structure is unknown. But one critical artifact survived: a stone sarcophagus of a design peculiar to the Kansai region, hundreds of miles to the southwest. This is the signature of the Yamato kingdom, the dominant political power of the era, and its presence in a tomb this far east raises compelling questions. A Shinto shrine dedicated to Tenjin once occupied the narrow neck between the circular and rectangular sections, giving the kofun its name -- Tenjinyama, the mountain of the heavenly deity.

Who warranted a tomb of this scale in the Kanto frontier? Scholars have debated the question for decades. The sheer size of the Ota Tenjinyama Kofun -- the only burial mound exceeding 200 meters in eastern Japan -- suggests someone who controlled the entire ancient Keno Province. The Kansai-style sarcophagus points to a direct connection with the Yamato court, but the nature of that connection remains uncertain. The occupant may have been a local king allied with Yamato, an opponent who adopted its burial customs, a provincial governor (Kuni no miyatsuko) appointed by the central court, or even a military commander dispatched to rule the eastern provinces. The Nihon Shoki, Japan's oldest official history, mentions the Keno clan as active participants in Yamato diplomacy with the Korean Peninsula, placing them squarely in the geopolitical currents of the 5th century. Whatever the answer, subsequent burial mounds in the region are noticeably smaller and scattered among sub-regional clusters -- suggesting that the power consolidated by this one ruler fragmented after their death.

The Ota Tenjinyama Kofun does not stand alone. Approximately 300 meters away lies the Nyotaizan Kofun, a large scallop-shaped tumulus with the same orientation and approximate construction date, separately designated as a National Historic Site. Paired tumuli of this kind -- one keyhole-shaped, one scallop-shaped -- appear at other major burial complexes across Japan, including the Saitobaru Kofun group in Miyazaki Prefecture on the southern island of Kyushu. To the north sits a smaller circular mound, 36 meters in diameter and 3.2 meters high, that yielded the same types of haniwa, confirming it as a contemporary satellite tomb. Together, these structures formed a funerary landscape that declared the power and continuity of a ruling lineage across the Kanto plain. Designated a National Historic Site in 1941, the complex sits just a 15-minute walk from Ota Station on the Tobu Railway, where the modern city hums around the silent earthworks of a forgotten dynasty.