Mt. Aso Nakadake Crater — Active Volcano You Can Approach

Mt. Aso is one of the world's largest active calderas (25km across) containing five volcanic peaks. Nakadake is the only peak with an accessible active crater — visitors can walk to the crater rim and look directly into the turquoise acidic lake 100 meters below, which constantly emits sulfurous gas. The crater's volcanic activity level determines access: when gas levels are safe, the viewing platform is open; during heightened activity, access closes.

The caldera itself was formed 90,000 years ago and now contains towns, farms, and forests — a rare example of humans living inside an active volcanic system.