Maruyama Kofun

Because of its name, the mound was long considered a simple enpun (円墳), a circular tomb, but archaeological surveys have found that it is an extremely large zenpō-kōen-fun (前方後円墳), shaped like a keyhole with one square end and one circular end when viewed from above. It is built on a slope on a hill and has a total length of 318 meters. The anterior rectangular portion has a front width of 210 meters and a height of 15 meters; the posterior circular mound has a diameter of 155 meters and a height of 21 meters. No fukiishi roofing stones have been found on the mound, and it is surrounded by a single moat.

The horizontal-entry burial chamber is 28.4 meters long. The 20.1 meter entrance passage is covered with six huge natural stones, each 4.8 meters long, over 1 meter wide and about 1.5 meters in thickness. The chamber itself is 8.3 meters long, 4.1 meters wide at its widest point and 4.5 meters high. Burial chambers are usually placed in the center of a circular mound, but here it lies about 20 meters off center.

Inside are two hollowed-out house-shaped stone coffins, placed perpendicular to each other in an L shape. The inner coffin has a lid 2.42 meters long, 1.44 meters wide and 0.42 meters high, and the front coffin has a lid 2.75 meters long, 1.41 meters wide and 0.63 meters high. The material is rhyolite welded tuff and Tatsuyama stone from the Kakogawa area. About one meter of soil has accumulated in the chamber, and details of the bodies in the coffins are unknown.

Several investigations of the interior of the burial chamber were carried out from the late Edo period to the Meiji period. In his 1796 Koretoku Jikkenki, Tsutsumi Koretoku measured the length of the chamber and recorded that it was flooded with water up to his waist, which was also recorded in Kitaura Sadamasa's 1848 Utsuminawa. According to Tsukui Kiyokage's 1854 Seiseki Zushi, two house-shaped stone coffins remain inside, one parallel to the main axis toward the right of the chamber and one at the back facing to the side.

In the Meiji period, William Gowland, an English mining engineer who has been called the Father of Japanese Archaeology, described it as Japan's largest dolmen. According to his records, the passageway is covered with six huge natural stones, the largest being 16 feet, and was about 60 feet long, 8 to 10 feet high and 4 to 8 feet wide, with walls of huge rough natural stones. After walking 40 feet down the passageway he found that water had accumulated to a depth of about four feet, so he could not enter the main chamber, but he observed two house-shaped stone coffins barely emerging from the water. Many of the items he excavated from the site are kept in the British Museum.

In 1880 a document called the Afukino Sanryoki was discovered at Kozan-ji in Kyoto. It detailed the state of the interior of the tomb and stated that the tomb had been robbed during the Kamakura period.

In 1991 a child living in Kashihara City, playing with a friend, discovered the entrance to the horizontal burial chamber passageway outside the fence of the mound. The opening had been exposed when the soil of the mound collapsed after heavy rain a few days earlier. The child's father entered the mound through the passageway and took photographs of the chamber interior. These were analyzed jointly by Tokai University's Information Technology Center and Konica. Both house-shaped sarcophagi were arranged as recorded in Edo period sources and were buried in mud up to the lids.