Mount Aso: The Land of Fire

Mount Aso (Aso-san) is not just a mountain; it is a geographic colossus. Located in the center of Kyushu, Japan’s southern island, it boasts one of the largest populated volcanic calderas in the world. The caldera measures a staggering 25 kilometers north-to-south and 18 kilometers east-to-west, with a circumference of over 100 kilometers.

Unlike most volcanoes where people live on the slopes, at Aso, people live inside the volcano. The caldera floor is a sprawling plain home to nearly 70,000 people, containing three towns (Aso, Takamori, and Minamiaso), abundant rice paddies, and two railway lines. From the center of this flat basin rises the central volcanic group, the Aso Gogaku (Five Peaks of Aso), creating a landscape that looks like a fortress built by giants.

Geological Setting: The Super-Eruptions

The Aso we see today is the result of four massive caldera-forming eruptions that occurred between 90,000 and 270,000 years ago.

• Aso-4: The largest of these, occurring about 90,000 years ago, was truly cataclysmic. It ejected over 600 cubic kilometers of volcanic material. Pyroclastic flows raced across the sea to neighboring islands, and ash from this event has been found as far away as Hokkaido in northern Japan.
• The Five Peaks: The central cone complex consists of five distinct peaks:
  1. Mt. Nakadake (1,506 m): The only currently active peak. It is the one tourists flock to see.
  2. Mt. Takadake (1,592 m): The highest point.
  3. Mt. Nekodake: The jagged, saw-toothed peak that looks completely different from the others.
  4. Mt. Eboshidake: Characteristics for its gentle slopes.
  5. Mt. Kishimadake: Located near the tourist center.

Mythology: The Kick of the God

The geography of Aso is so unique that local legend attributes it to a divine act of civil engineering.

• The Lake: Legends say the caldera was originally a giant lake, trapping the people and preventing them from farming.
• The God: The deity Takeiwatatsu-no-mikoto (grandson of the first Emperor Jimmu) wanted to create more farmland. He decided to drain the lake.
• The Kick: He tried to kick a hole in the crater wall. Ideally, he would have kicked the easiest spot, but he stumbled. On his second attempt, he sat down and kicked the wall at Tateno with all his might. The wall shattered, the water rushed out (creating the Shirakawa River), and the fertile plain of the caldera floor was revealed.
• Nekodake’s Punishment: Why does Mt. Nekodake look so jagged? Legend says Nekodake was the youngest brother of the five peaks and liked to brag about being the tallest. The other brothers complained to the god Takeiwatatsu. The god slapped Nekodake on the head to humble him, shattering his summit into the saw-toothed shape we see today.

The Millennium Grasslands

One of the most striking features of Aso is not the grey ash, but the emerald green grass. The rolling hills of the caldera (especially Kusasenri) are covered in lush grasslands that have been maintained by humans for over 1,000 years.

• Noyaki (Controlled Burning): Every spring (usually in March), locals perform Noyaki. They deliberately set fire to the grasslands. This 1,000-year-old tradition prevents the forest from taking over, kills harmful insects, and produces fresh ash that fertilizes the new grass.
• Akaushi Beef: These grasslands are the grazing ground for the Japanese Brown Cattle (Akaushi). Unlike the fatty, marbled Wagyu (Black cattle), Akaushi are lean and muscular because they roam freely on the steep volcanic slopes. The result is a healthy, red meat that is the signature dish of the region (Akaushi-don).
• GIAHS Status: In 2013, this sustainable system of agriculture was designated as a Globally Important Agricultural Heritage System (GIAHS) by the FAO.

Mount Nakadake: Staring into the Abyss

The Nakadake Crater is one of the few active volcanoes in the world where you can look directly into the conduit… when it behaves.

• The Blue Lake: When activity is low, a stunning steaming turquoise lake forms in the bottom of the crater (Crater 1). It is extremely acidic (pH near 0) and hot (around 60°C).
• The Gas: The volcano constantly emits large amounts of sulfur dioxide (SO2). The access to the crater rim is controlled by a strict light-signal system (Blue, Yellow, Red). If the gas concentration spikes, sirens wail, and the area is evacuated immediately. Asthma sufferers are often banned from visiting the rim even on “safe” days.
• The Sand: Volcanic ash is a daily reality. Concrete shelters are scattered around the crater rim for tourists to dive into in case of a sudden eruption.

Aso Shrine and the 2016 Earthquake

The Aso Shrine, located on the caldera floor, is one of the oldest in Japan, dating back to 281 BC. It is dedicated to the creator god Takeiwatatsu.

• The Disaster: On April 16, 2016, a magnitude 7.0 earthquake struck Kumamoto. The violent shaking caused the shrine’s magnificent two-story Romon Gate (a National Treasure) and the main worship hall (Haiden) to collapse completely.
• The Resurrection: The sight of the collapsed shrine devastated the local community. However, a massive restoration project began immediately. Using traditional carpentry techniques, artisans have been piecing the shrine back together like a giant puzzle. The Haiden was completed recently, and the Romon Gate is nearing full restoration—a symbol of Aso’s resilience.

Tourism: Fire and Water

Aso represents the duality of nature: the fire of the volcano and the water of the springs.

• Kusasenri-ga-hama: A postcard-perfect location. It is a double crater of an extinct volcano (Eboshidake) now filled with a vast green meadow and two rain ponds. Horses graze lazily against the backdrop of the smoking Nakadake plume.
• Shirakawa Spring Source: Remember the water that drained when the god kicked the wall? It flows underground and re-emerges here. This spring pumps out 60 tons of water per minute. The water is so pure it is bottled and sold across Japan.
• The Road to Laputa: The “Road of Laputa” (named after the Ghibli movie) was a scenic path that seemed to float in the clouds on the outer rim. Sadly, it was severely damaged by the 2016 earthquake and remains closed, a reminder of the earth’s shifting power.

The Road of Laputa: A Lost Icon

One of Aso’s most famous modern legends is not ancient, but cinematic. The “Road of Laputa” (official name: Aso City Road Kario Route) became an internet sensation for its resemblance to the floating island in the Studio Ghibli movie Castle in the Sky.

• The View: The road runs along the ridge of the outer crater rim. When the “Sea of Clouds” (unkai) fills the valley below, the road appears to float in the sky, disconnected from the earth.
• The Damage: Tragically, the 2016 Kumamoto earthquake caused massive landslides that severed the road in multiple places. It remains closed to traffic, a crumbling ruin that has ironically become even more mythical because it is now unreachable.

Uchinomaki Onsen: The Hot Spring Town

Inside the caldera, Uchinomaki is the largest hot spring resort.

• Literary History: It was a favorite retreat for great Japanese writers like Natsume Soseki and Yosano Akiko. Soseki’s novel The 210th Day is set here, depicting a hike up the mountain during a storm.
• The Water: The town has over 30 different hot spring sources. It is unique because the water comes from straight, vertical pipes driven into the ground, rather than pumped.

The Aso Geopark: Learning from the Volcano

In 2009, Aso was certified as a Japanese Geopark, and later as a UNESCO Global Geopark. This status isn’t just a label; it’s a commitment to education.

• The Aso Volcano Museum: Located Kusasenri, this museum offers a real-time feed from cameras installed inside the active crater. It is the safest way to “see” the lava when the crater itself is closed due to gas.
• Geo-Sites: The park designates specific “geo-sites” like the Komezuka cone (a perfect scoria cone that looks like an inverted rice bowl) and the Tateno Barranco (the gap in the caldera wall). Legends say Komezuka has a dimple on top because the god Takeiwatatsu scooped out a handful of rice to give to the poor.

The Trains of the Caldera: Aso Boy! and the Switchback

The railway journey into the caldera is an attraction in itself.

• The Switchback: The JR Hohi Main Line has to conquer a massive elevation change to get from the outer rim to the caldera floor. It does this using a three-stage switchback at Tateno Station. The train literally zig-zags back and forth up the steep slope, allowing passengers to see the driver change ends of the train. It is a rare feat of engineering that thrills train enthusiasts (densha otaku).
• Aso Boy!: This is one of Kyushu’s most famous “D&S” (Design and Story) trains. It is a family-oriented train themed around a fictional dog character named “Kuro-chan”. The train features a “white” car with a ball pool for children, special bento boxes, and panoramic windows to view the smoking craters. Riding this train across the caldera floor is a surreal mix of whimsical fun and raw volcanic power.

Conclusion

Mount Aso is a place where humanity and geology are locked in a surprisingly harmonious embrace. For millennia, people have burned the slopes, worshiped the angry peaks, and eaten the food grown in the volcano’s shadow. It is a landscape of immense scale, where the horizon is defined not by the sky, but by the walls of a crater that seems too big to be real.