Ultra-Trail Mt. Aso Round Trail is a 100km ultra marathon in Aso, Japan. Distance: 100km. Elevation: 4,400m D+. Difficulty: moderate (Challenge Rating 47/100). DNF rate: 15%. Time limit: 24h. Surface: trail. Established 2014.
Ultra-Trail Mt. Aso Round Trail
Aso, Kumamoto, Japan
"100km around a massive volcanic caldera. Grassy ridgelines, active craters, and Japanese mountain beauty."
Run the rim of one of the world's largest volcanic calderas — 100km around Mount Aso with views into the smoking crater.
The Course
A loop around the outer rim of Mount Aso's caldera — one of the largest volcanic calderas in the world. The course traverses grassy ridgelines, volcanic terrain, and Japanese mountain forests with views into the active volcanic crater.
mountainous
loop
8
Runner Experience
Standard aid station support with food, drinks, and basic medical assistance at regular intervals.
Warm, inclusive finish line with a strong community feel. Runners celebrate together regardless of finish time.
Race Day Weather
16°C
8° – 22°
65%
30%
Japanese spring — pleasant temperatures with occasional rain. Morning mist in the caldera is common.
Stats & Records
2014
500
16:00:00
Entry
¥18,000
December
weeks
Lore & Trivia
Ultra Trail Mt Aso runs through the caldera of Mount Aso on the island of Kyushu — the largest active volcanic caldera in the world, so vast that entire towns exist inside it. The course crosses volcanic grassland, cedar forest, and the rim of the caldera with views of the smoking Nakadake crater. Japanese trail running culture brings a reverence to volcanic landscapes — Aso is both sacred and dangerous, and running through its caldera carries a weight that ordinary mountain races don't.
- ▸ Mount Aso's caldera is 25km wide — one of the largest in the world, and runners circuit its entire rim.
- ▸ The active volcano at the center of the caldera occasionally emits sulfuric gas, adding a surreal element.
- ▸ The wide grassy ridgelines of Aso are unlike typical Japanese mountain terrain — they feel almost alpine.
- ▸ Aso is home to wild horses that graze on the caldera grasslands alongside the trail.
- ▸ Established in 2014.
Mount Aso is the largest active volcanic caldera in the world. Running inside it is running inside a living volcano.
Compare with another race
Similar Races
28 JUN 2026
Lake Saroma 100km Ultramarathon
Saroma, Japan
6 MAY 2026
Shiga Kogen Extreme Trail
Shiga Kogen, Japan
24 APR 2026
Ultra-Trail Mt. Fuji
Kawaguchiko, Japan
24 APR 2026
Ultra-Trail Mt. Fuji
Kawaguchiko, Japan
Frequently Asked Questions
How long is Ultra-Trail Mt. Aso Round Trail? +
Ultra-Trail Mt. Aso Round Trail is a 100km (100km) ultra marathon with 4,400m of elevation gain. The course is loop on trail terrain.
How hard is Ultra-Trail Mt. Aso Round Trail? +
Ultra-Trail Mt. Aso Round Trail is rated moderate with a Challenge Rating of 47/100 (Challenging). The DNF rate is 15%. Runners have 24 hours to finish.
What is the weather like at Ultra-Trail Mt. Aso Round Trail? +
Average temperature is 16°C (range 8°C to 22°C) with 65% humidity and a 30% chance of rain. Japanese spring — pleasant temperatures with occasional rain. Morning mist in the caldera is common.
Are crew and pacers allowed at Ultra-Trail Mt. Aso Round Trail? +
No crew access is permitted. Pacers are not allowed. Mandatory gear is required.
What is the average finish time for Ultra-Trail Mt. Aso Round Trail? +
The average finish time is 16:00:00. About 500 runners finish each year. The DNF rate is 15%.
Are you the organizer? Claim this race
Stay in the loop
New races, course records, and stories from the trail — monthly