ASUKA, Nara -- Remains of a passage have been newly discovered at the ruins of the Koyamada tomb here, the Archaeological Institute of Kashihara has announced, adding it has confirmed the tomb is among the largest of its kind.

In January 2015, the institute announced that a stone ditch, believed to be a moat, had been uncovered at the site in the village of Asuka during the reconstruction of a special needs school. The ditch was roughly 48 meters long, 7 meters wide at its top, 3.9 meters wide at its bottom and about 1 meter deep. An unusual construction of rock slabs laid like a staircase on the upper part of the tomb also attracted attention.

The ditch is thought to have lain on the northern side of the tomb. In December last year, the institute started excavation at a point around 60 meters south of the ditch. It found a site where two large rocks about 2.6 meters apart and measuring about 1.5 meters east to west and more than a meter north to south once lay. A stone ditch, about 20 centimeters wide and about 30 centimeters deep, was found between the place where the two rocks were situated. The rocks are thought to have been foundation stones for the two walls of a corridor linking the inside of the tomb with the outside, and the stone ditch is thought to have been a drainage ditch.

Around 10 meters to the north, excavators found another site where another large rock once lay, showing that the corridor extended to the north.

Considering the corridor as the central axis of the tomb, and calculating from what has been estimated as the tomb's northwest corner through the excavations so far, the institute has estimated the length of one side of the tomb at around 70 meters, longer than the longest side of the Yamada Takatsuka tomb that is thought to be the resting place of Empress Suiko, which extends around 60 meters. It is also longer than the length of the Ishibutai tomb thought to be where Soga no Umako was interred, which runs about 50 meters.

Additionally, pieces of circular tile thought to date back to around the year 630 were unearthed from the tomb's foundation soil. Combined with the fact that the stone slabs on the upper part of the tomb resemble the Dannozuka tomb where Emperor Jomei was reinterred in 643, researchers at the institute have estimated the Koyamada tomb's construction date at around 640. Based on the dating of pottery buried in the stone ditch, the tomb is thought to have been abandoned by the late 7th century, which would mean that it was no longer being used within a few decades of its construction.

Regarding who was interred at the Koyamada tomb, Fuminori Sugaya, head of the institute, says, "Emperor Jomei, who was first interred in 642, is a candidate." Meanwhile, at least one other researcher thinks it is a tomb built the same year for Soga no Emishi, another ruler with power on par with that of the emperor.

The site is currently reburied, and no tours of it are being given.