Aso-san comprises the world's largest caldera—128 km (80 miles) in circumference, from 18 km to 24 km (11 to 15 miles) wide in places—formed after a massive lava-dome collapse some 100,000 years ago. Inside the crater are seven settlements, not to mention herds of cows and horses. The emerald-green grasses that nourish them thrive in the fertile volcanic soil. The crater area, officially named the Aso-Kuju (Mt. Aso
National Park), contains five volcanic cones; one is the still-active Naka-dake, and it sticks up out of the side of the taller Taka-dake just east of the crater's center. There's no mistaking the sulfurous stench of the mighty belches that gust freely from its mouth.