Mount Aso is an active volcano located in **Japan**, specifically on the island of **Kyushu**. It is one of the most famous volcanoes in the country and is known for having one of the **largest calderas** (a large volcanic crater) in the world, measuring about **120 kilometers in circumference**.

Mount Aso is not only large but also **active**, meaning it still erupts from time to time. Its most active crater is called **Nakadake**, which sometimes emits smoke, gas, and ash. Because of this activity, Mount Aso is closely monitored by Japanese scientists.

The area around the volcano is also popular with tourists. People visit to see the crater, enjoy hot springs (called **onsen**), and explore the surrounding national park. The volcanic soil is very rich, which makes the land good for farming.

Japan is a country that has many volcanoes because it lies on the **Ring of Fire**, a zone around the Pacific Ocean where many earthquakes and volcanic eruptions happen. Mount Aso is just one of many active volcanoes in Japan, but it is especially well-known because of its size and frequent activity.

So, the correct answer is **Japan**, the country where Mount Aso is located.