What Is the Aso Caldera? The Five Peaks and Four Giant Eruptions
This guide is for visitors who want to understand what they are looking at in Aso. It explains the caldera, the five peaks and the four giant eruptions that shaped the land. It also explains why people live inside an active volcano. The explanations come from the Aso Volcano Disaster Prevention Council and Kumamoto Prefecture.
We used the official English pages of the Aso Volcano Disaster Prevention Council, the Kumamoto Prefecture tourism site and the city of Aso. All pages were read on 2026-10-04.
What is a caldera?
The council explains that a caldera is a circular depression larger than an ordinary crater. Most craters do not exceed one kilometer across, it says, but a caldera is much larger. Unlike a crater, a caldera is not thought to form from a simple eruption or explosion.
It names three types: explosive, non-explosive and non-volcanic. The council classes Aso as a non-explosive caldera. It adds that the word comes from the Portuguese word for cauldron.
How big is it?
The council says Aso is one of the largest calderas in the world. It gives the size as 17 kilometers east to west and 25 kilometers north to south, with an area of roughly 350 square kilometers. It says Aso is about equal in size to the Aira caldera.
The prefecture gives a different east-west figure. Its page says the caldera stretches 25 kilometers north to south and 18 kilometers east to west. We use the council's figures in the rest of this guide, and note the difference here.
The five peaks
The council says "Mount Aso" is the name for a central group of craters, which includes the still-active Mount Naka. It stresses that there is no single Mount Aso. The five Aso peaks are:
- Mount Taka
- Mount Naka
- Mount Eboshi
- Mount Kijima
- Mount Neko
The prefecture calls them "Aso Gogaku" and says the range, seen from the north, is said to look like a sleeping Buddha. The council adds that the outer mountains formed from pyroclastic flows outside the caldera, and are also part of the Aso volcano in academic terms.
Four giant eruptions
The council says the present Aso was created by four large eruptions in the ancient past. It gives the dates as about 270,000, 140,000, 120,000 and 90,000 years ago.
The last of these was especially large. The council says its flows crossed the sea and left deposits at Shimabara, Amakusa and in Yamaguchi Prefecture, as far as Akiyoshidai. Volcanic ash more than 10 centimeters deep remains in eastern Hokkaido.
So much magma left the ground that a large underground space formed and the ground collapsed. The council says this is thought to be why the caldera exists. About 70,000 years ago, after the caldera formed, the central group of craters rose in its middle.
Living inside an active volcano
The prefecture says what makes Aso almost unique is that people cultivated the once infertile land inside the caldera. They built roads, railways, farms and houses there. In its words, people are living and thriving inside an active volcano.
The prefecture lists several benefits of the volcanic land:
- Hot springs. It says there are countless onsen in Aso, each with different water.
- Groundwater. It says the soil stores rainwater, and that all water used for daily life in Aso comes from natural groundwater.
- Food. It names grass-fed Akaushi beef, "dengaku" grilled over a sunken hearth, and a local pickled mustard leaf called "takanazuke".
- Aso Shrine. It says the shrine has more than 2,000 years of history, and that local people have long respected the volcano as a god.
The prefecture also suggests seeing the caldera from a hot-air balloon or a paraglider, and says a guided English tour is available through the Aso Geopark. We could not confirm current tour availability.
The cloud sea season
The city of Aso's front page notes that the cloud sea season has arrived. Its "Cloud Sea Challenge" started on October 1, and the city posts a daily result on its video channel. We did not check any forecasts for cloud sea conditions.
Before you plan a climb, read our guide on whether you can visit the Aso crater now.
What we could not verify, and therefore left out
- Which caldera size is right: the council gives 17 km by 25 km, and the prefecture gives 18 km by 25 km.
- Current availability and prices of balloon, paraglider and guided tours.
- Cloud sea forecasts and the best viewing places for the season.
- Opening hours and fees at Aso Shrine.
Related guides
- How do you get to Aso and the crater road?
- Can you visit the Aso crater now? Alert level 2, gas and the mountain road
Sources
All URLs retrieved 2026-10-04.
- Description of Mt. Aso (Aso Volcano Disaster Prevention Council) — (caldera definition; size; five peaks; four eruptions)
- Welcome to Aso! (Kumamoto Prefecture tourism site, archive page) — (people living in the caldera; hot springs; groundwater; food; shrine)
- Aso City official site, front page (in Japanese) — (cloud sea season, 2026-10-01 and 2026-10-02 notices)