The famous “blue fire” of Mount Ijen is a rare natural event: It is not lava glowing blue, but actually burning sulfuric gases. Sulfur gases emerge from cracks in the volcano at extremely high temperatures (over 600°C / 1,112°F). When these gases come into contact with oxygen, they ignite, producing bright electric-blue flames. Some of the burning sulfur condenses into liquid and flows, creating the illusion of blue lava. This phenomenon is only visible in darkness, which is why visitors hike at night to witness it.