As the name "Maruyama" suggests, this tumulus has long been considered a simple enpun (円墳)-style circular tomb, but archaeological surveys have found that it is an extremely largezenpō-kōen-fun (前方後円墳), which is shaped like a keyhole, having one square end and one circular end, when viewed from above.

In the Meiji period, William Gowland, an English mining engineer who has been called the "Father of Japanese Archaeology".,[3] evaluated it as "Japan's largest dolmen ".

[2] Although tradition had stated since ancient times that this was the joint tomb of Emperor Tenmu and Empress Jitō no evidence had been uncovered to verify that this despite these numerous investigations.

As a result, the Imperial Household Ministry determined that the joint tomb of the two emperors was officially the Noguchi no Omune, located about one kilometer southwest, although part of the upper part of the circular mound was designated as a "reference site" for imperial tombs, and placed under restrictions against further archaeological investigation.

During an on-site survey by the Imperial Household Agency, a fragment of Sue ware pottery, was unearthed, and since not a single haniwa, which is usually found in mounds built up until the mid-5th or 6th century, was found, the theory that the mound was built in the latter half of the 6th century is considered the most likely.

In 1991, while playing with a friend, a child living in Kashihara City discovered the entrance to the horizontal burial chamber passageway outside the fence of the mound.

The hole was exposed when the soil of the mound collapsed due to heavy rain a few days earlier.

The photos were analyzed by a joint effort between Tokai University's Information Technology Center and Konica.