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Ijen UNESCO Global Geopark is characterized by a high concentration of volcanic cones, craters, and lava flows, showcasing the complex geological processes that have shaped the region over millions of years. The most prominent attraction within the geopark is Ijen volcano and its carter lake, which is famous for being the most acidic lake in the world, and its otherworldly blue fire phenomenon—a rare sight caused by ignited sulfuric gases that emerge from cracks in the crater. Ijen is one of the most active volcanoes in the Ijen caldera system. Approximately 22 post-caldera volcanic cones were generated inside and on the rim of the caldera. This volcanic activity grows above the limestone basements and tertiary volcanic rock which can be found as the old hills 50 km south of Ijen. The Ijen Crater lake is a remarkable natural feature influenced by a range of geological processes. It contains chemical solutions resulting from the interaction of magmatic volatiles, rock-fluid interactions, evaporation, dilution by meteoric water, and the recycling of water through subsurface hydrothermal systems. Acting as a condenser for volatile water from shallow magmatic heat sources, the lake assimilates acidic magmatic vapours, which leads to the formation of highly acidic water. Notably, the Ijen Crater exhibits a distinctive blue fire phenomenon, attributed to the reaction between sulfur and oxygen. While most volcanoes produce red or orange fire, the high sulfur concentrations and temperatures exceeding 360°C in Ijen Crater create a unique environment for the formation of blue fire. This phenomenon is observable only during night-time due to the overpowering intensity of sunlight obscuring the visibility of the blue flames.