The Magnetic Rocks of Susa Koyama: A Natural Wonder in Yamaguchi Prefecture

Perched atop Mount Koyama in the Susa district of Hagi City, Yamaguchi Prefecture, lies one of Japan's most unusual geological treasures. The Magnetic Rocks of Susa Koyama (須佐高山の磁石石, Susa Kōyama no Jishaku-ishi) are massive gabbro boulders that possess an extraordinarily powerful natural magnetism — so strong that they can deflect a compass needle and even cause it to reverse direction within just a few centimeters. Designated as a National Natural Monument in 1936, these remarkable rocks offer visitors a rare opportunity to witness the invisible forces of nature made tangible, set against the sweeping panoramic views of Susa Bay and the Sea of Japan coast.

What Are the Magnetic Rocks?

Mount Koyama (高山, read "Kōyama") rises to an elevation of 532.8 meters above sea level on the northeastern edge of Susa Bay. The mountain is composed almost entirely of gabbro, a dark-colored intrusive igneous rock that formed approximately 13 million years ago when magma slowly cooled and solidified deep underground. Over geological time, the overlying layers eroded away, exposing the gabbro on the surface.

What makes the summit boulders extraordinary is their abnormally strong magnetism. If you bring a compass near these rocks, the needle is visibly attracted and deflected. Move the compass just 10 to 15 centimeters, and the needle may swing in the completely opposite direction. Even small fragments of rock scattered on the ground can cause a compass needle to react strongly. The magnetic intensity is greatest near the summit and gradually weakens as you descend the mountain.

Remarkably, the rock itself is not magnetite — a mineral commonly associated with natural magnetism. The prevailing scientific theory is that repeated lightning strikes over thousands of years sent massive electrical currents through the gabbro, magnetizing the iron-bearing minerals within it. This phenomenon of non-magnetite rock acquiring such powerful magnetic properties is exceptionally rare worldwide and gives these rocks their great scientific significance.

Why Were They Designated a Natural Monument?

The Magnetic Rocks of Susa Koyama were designated as a National Natural Monument (天然記念物) on December 16, 1936 (Shōwa 11). The official designation document noted that the quartz gabbro boulders exposed on the summit of Mount Koyama possess magnetism as strong as natural magnets, and that it is extremely unusual for non-magnetite rock to exhibit such powerful magnetic properties. This rarity, combined with the scientific value of studying how lightning-induced magnetization occurs in igneous rock, earned the site its protected status.

The designation encompasses the areas known as Takayama Minamidaira and Takayama Kitadaira on the summit, protecting a broad zone of magnetically charged rock outcrops. As a nationally designated natural monument, the rocks are preserved under Japanese cultural property law, ensuring this extraordinary geological phenomenon remains intact for future generations of researchers and visitors alike.

The Legend of Susanoo and the Magnetic Rocks

The Susa area draws its very name from Susanoo (須佐之男命), one of the most important deities in Japanese mythology. According to local legend, when Susanoo was navigating the seas and lost his bearings, a lodestone aboard his vessel was mysteriously drawn toward the magnetic rocks of Mount Koyama. Following this pull, Susanoo was guided safely into Susa Bay, where he made landfall. The area has been known as Susa in honor of the deity ever since.

This mythological connection adds a layer of cultural richness to the geological phenomenon. The Kōtei-sha Shrine (黄帝社), located partway up the mountain, was historically revered by seafarers. During the era of kitamae-bune (northern trade ships), sailors would pay their respects at this shrine before setting out across the Sea of Japan. Votive ship paintings (船絵馬) offered to the shrine reflect centuries of maritime faith, and 49 of these paintings were designated as a National Important Tangible Folk Cultural Property in 2010.

Highlights and Things to See

Experiencing the Magnetism Firsthand

The most memorable part of any visit is bringing a compass to the summit rocks and watching the needle behave erratically. As you move the compass across different parts of the rock surface, the needle swings, dips, and reverses — an invisible force made visible. Visitors are encouraged to bring their own compass (available at most outdoor supply stores) to fully appreciate this natural wonder. The sensation of holding a compass that refuses to point north is both educational and genuinely thrilling.

Panoramic Views from the Summit

The mountaintop observation platform offers sweeping views of Susa Bay, the Sea of Japan coastline, and the layered mountain ridges of the San'in region. On clear days, the contrast between the deep blue sea and the lush green mountains is spectacular. The summit also features a first-order triangulation point and the only astronomical survey point (天測点) in Yamaguchi Prefecture, adding to its scientific interest.

Wind Caves (風穴)

Just below the summit on the northern slope, you can find wind caves — natural openings in the volcanic rock that emit cool air, much like the similar formations at nearby Mount Kasayama. These provide a refreshing break during summer visits.

Kōtei-sha Shrine and the Votive Ship Paintings

Before reaching the summit, a trail branches off toward the Kōtei-sha Shrine, dedicated to the legendary Chinese emperor Huangdi. This unusual shrine features stone guardian dogs whose pedestals are shaped like ships, facing westward toward the Asian continent. The nationally designated votive ship paintings housed here document centuries of maritime history and trade along the Sea of Japan coast.

Nearby Attractions

The Susa district of Hagi City is a geological wonderland, and the Magnetic Rocks are just one of several extraordinary natural sites in the area. In 2018, the broader Hagi Geopark — which includes Mount Koyama and the surrounding area — was officially certified as a Japanese Geopark, recognizing the region's outstanding geological heritage.

Susa Hornfels

Just a 15-minute drive from Mount Koyama, the Susa Hornfels is a stunning sea cliff displaying alternating bands of gray-white sandstone and black shale. These striped cliffs, standing approximately 12 meters tall, were formed by the same magmatic event that created the gabbro of Mount Koyama. Selected as one of Japan's Top 100 Geological Sites and also recognized among the Landscape Heritage of the 21st Century, this is one of Yamaguchi Prefecture's most photogenic coastal landmarks. Visitors can descend via a walking path to view the cliffs from below.

Susa Bay Geo-Cruising

From approximately late April through October, local fishermen operate sightseeing cruises aboard squid-fishing boats that bring passengers close to the dramatic sea cliffs of Susa Bay. The bay itself is designated as both a National Scenic Beauty and Natural Monument, and viewing the hornfels formations from the water offers a completely different perspective.

Tatami-ga-fuchi and Ryūrinkyō

Further inland, Tatami-ga-fuchi features hexagonal columnar basalt joints along a riverbed that resemble a pavement of turtle shells. Nearby Ryūrinkyō (Dragon Scale Village) displays striking basalt columns that look like the scales of a mythical dragon, and is designated as a Yamaguchi Prefectural Natural Monument. Both sites are part of the Hagi Geopark.

Susa Male Squid (須佐男命いか)

Susa is famous for its premium squid, branded as "Susa Mikoto Ika" after the deity Susanoo. Local restaurants serve this delicacy as sashimi so fresh it is still translucent, offering a culinary experience unique to this fishing town. The Ika Marché at JR Susa Station is a great place to pick up local seafood and specialty products.