Masuda Iwafune is a gigantic stone structure located in Shira-kashi-cho, Kashihara City, Nara Prefecture. Designated a Historic Site by Nara Prefecture, it is one of the Asuka region’s best-known ancient stone monuments, alongside the Turtle Stone and Sakafuneishi. Among these remarkable structures, Masuda Iwafune is particularly impressive for its enormous size and mysterious purpose.
The monument stands near the summit of Mount Iwafune, at an elevation of approximately 150 meters, in a hilly area west of Shira-kashi Minami Elementary School within Kashihara New Town. Its shape resembles a trapezoid, measuring approximately 11 meters from east to west, 8 meters from north to south, and 4.7 meters in height.
It is made of hard quartz diorite and consists of an enormous block of rock with steep, almost vertical sides. Its scale and carefully worked surfaces make it an extraordinary example of ancient stone craftsmanship.
A groove approximately 1.6 meters wide runs from east to west across the upper surface and down part of the side. Two square holes, each measuring about 1.6 meters on each side and 1.3 meters deep, have been carved into this groove. The holes are separated by approximately 1.4 meters, and traces suggest that the work was abandoned before the structure was completed.
Near the bottom of the rock, a grid of shallow grooves approximately 10 centimeters deep can also be seen. These marks are thought to be part of an unfinished process intended to smooth and finish the stone surface.
No definitive historical records explain when Masuda Iwafune was built or what it was intended to do. Based on its dimensions and the techniques used to shape the rock, it is generally thought to date from around the seventh century. Several theories have been proposed to explain its original function.
One longstanding theory is that the rock was intended to serve as the base for a stone monument commemorating Masuda Pond. According to this theory, the inscription was erected in 822, during the Heian period, and was later removed, leaving only its stone foundation behind.
Another theory suggests that pillars were intended to be placed in the two square holes, with a horizontal beam spanning them. Such a structure may have been used to observe celestial bodies, perhaps in connection with ancient astrology or the preparation of calendars.
Some researchers have proposed that the holes were designed to hold human remains after cremation and were meant to be covered with stone lids. Under this interpretation, Masuda Iwafune may have been associated with ancient burial customs and funerary rituals.
One of the leading theories is that Masuda Iwafune was intended to become an unfinished yokoguchi-shiki sekikaku, a stone burial chamber with a side entrance. Its structure resembles the side-entry stone chamber of Kengoshizuka Kofun, located approximately 500 meters to the southeast.
Several details support this interpretation. Water does not collect in the western hole, and the grid-like carvings on the sides appear consistent with work that was abandoned before completion. Although its original purpose remains uncertain, the monument continues to attract interest from archaeologists and historians.
By train: Masuda Iwafune is approximately a 20-minute walk from Okadera Station on the Kintetsu Yoshino Line. The site is about one kilometer west of the station.
By bus: From the west exit of Kashiharajingumae Station, take a Nara Kotsu bus bound for Minami-Shirakashi and get off at Minamimyohōjicho bus stop. From the trailhead, a steep uphill path leads to the monument.
Masuda Iwafune is a fascinating destination for anyone interested in archaeology, ancient history, or the mysteries of Japan’s past. The grooves and unfinished holes provide clues to the sophisticated stone-working techniques used by ancient craftsmen, even though the monument’s intended purpose remains unknown.
The elevated location also offers views across the Nara Basin, creating a memorable combination of historical intrigue and natural scenery.
The paths around Masuda Iwafune include steep slopes and uneven ground. Conditions can become particularly slippery during and after rain, so take extra care when climbing. Steps and ropes are installed near parts of the trailhead, but some sections remain difficult to navigate. Sturdy, comfortable walking shoes are strongly recommended.