This geopark is located in the Banyuwangi and Bondowoso Regencies in East Java Province. The geopark’s strategic location between the strait and the sea has made it a crossroads for human migration and commerce. Ijen is one of the most active volcanoes in the Ijen caldera system. About 22 post-caldera volcanic cones have formed both inside the caldera and on its rim. Ijen is the most acidic crater lake on Earth and the largest of its kind. Thanks to a rare phenomenon, high concentrations of sulphur rise from the active crater before igniting as they encounter the oxygen-rich atmosphere; as the gas burns, it forms an electric blue flame which is unique and only visible at night. The water itself is acidic because it is recycled by seeping into the volcanic subsurface.