Everything at Mount Ijen centres on the volcano. The essential experience is the blue fire — the otherworldly electric flames that burn at the volcanic vents before dawn, visible only in darkness. Reaching them requires a midnight start and the crater rim hike, then a descent into the crater itself. Once at the crater floor, you encounter both the Kawah Ijen acid lake — the world's largest — and the sulfur miners at work, one of the most intense human-labour environments still operating at this scale.
At sunrise, the crater rim offers extraordinary views over the turquoise lake and across the Ijen plateau to the coast. A gas mask is essential for crater descent; these can be rented at the Paltuding basecamp.