Mount Aso (阿蘇山 Aso-san?) is the largest active volcano in Japan, and is among the largest in the World. It stands in Aso Kujū National Park in Kumamoto Prefecture, on the island of Kyūshū. Its peak is 1592 m above sea level. Aso has one of the largest calderas in the world (25 km north-south and 18 km east-west). The caldera has a circumference of around 120 km (75 mi), although sources vary on the exact distance.
The central cone group of Aso consists of five peaks: Mt. Neko, Mt. Taka, Mt. Naka (also called Nakadake or Naka-Dake), Mt. Eboshi, and Mt. Kishima. The highest point is the summit of Mt. Taka, at 1592 m above sea level. The crater of Mt. Naka, the west side of which is accessible by road, contains an active volcano which continuously emits smoke and has occasional eruptions. Only the northernmost crater (the first crater) has been active for the last 70 years-1974, 1979, 1984-1985, 1989-1991, 2009, and 2011.
The present Aso caldera formed as a result of four huge caldera eruptions occurring over a range of 90,000–300,000 years ago. The caldera, one of the largest in the world, contains the city of Aso as well as Aso Takamori-cho and South Aso-mura. The somma enclosing the caldera extends about 18 km east to west and about 25 km north to south. Viewpoints from the somma overlooking the caldera are perched upon lava formed before the volcanic activity which created the present caldera.
Ejecta from the huge caldera eruption 90,800 years ago covers more than 600 km³ and roughly equals the volume of Mt. Fuji; it is presumed that the pyroclastic flow plateau covered half of Kyūshū.
Recently, the number of tourists visiting the area from Asian countries has been increasing. Previously, visitors could not drive their own vehicles up to the crater, but recently a car park was built a minute's walk away. A ropeway also provides access. At the foot of the mountain there are also various campsites, and horse riding at Kusasenrigahama. Modern poetry has been written about the beautiful scenery at Kusasenrigahama, which has been designated a place of scenic beauty, or natural monument.
Because Mount Aso is a volcano, there are many hot springs (Japanese:onsen) in the area. There are two hot springs within the crater that are associated with Aso Kujū National Park.