From today's Daily Yomiuri:
The mausoleum in Marukoyama in Asukamura, Nara Prefecture, is hexagonal, giving support to theories that have suggested the tomb belonged to a high-ranking prince, according to the municipal board of education.

The mausoleum is the first hexagonal tomb to be found in the Asuka area, where there are many tombs of Imperial and aristocratic family members that date back to the late sixth and early seventh centuries, when Empress Suiko (554-628) ruled the region.

Mausoleums were built in various shapes, but only a few hexagonal chambers have been discovered in the nation. Emperors, empresses and crown princes of the era usually were buried in octagonal mausoleums, so the Marukoyama mausoleum, which dates from the late seventh century, is believed to be that of a high-ranking prince rather than a crown prince, emperor or empress.

The discovery, which was announced Sunday, is expected to help researchers determine who was buried in the tomb.

Previously, archeologists reasoned that the mausoleum was a two-tiered circular tomb, but they discovered an angle of about 140 degrees within the chamber and then excavated the bottom tier on the west side. A similar discovery was made in 1977 on the tomb's north side. The archeologists now believe the mausoleum is hexagonal, with its longest side measuring about 24 meters.

In Taoism, an octagon represents the universe, while a hexagon symbolizes the world. Based on these ideas, the buildings of Chinese emperors were usually octagonal, whereas those of princes were usually hexagonal.

Several ancient mausoleums for such royalty as Emperor Tenji of Kyoto and Emperor Tenmu and Empress Jito in Asukamura are octagonal.

Before the discovery, experts reasoned from the size and location of the structure that the Marukoyama mausoleum belonged to Prince Kawashima (657-691), a son of Emperor Tenji, or Prince Yuge (?-699), a son of Emperor Tenmu.