It only takes an hour or two for a reasonably fit person to climb to the rim of Kawah Ijen, an active caldera in East Java. The views from the top are gorgeous, especially at sunrise, but the main rewards await you below, a 30-minute hike down a rocky path. Two of them, actually: the famous blue fire, rivulets of molten sulfur burning with a bright azure flame; and the pool of sulfuric acid filling the caldera, the largest acidic lake in the world. That's two unique geological phenomena in one hike - a pretty good deal!
You will see those wiry men pushing carts full of yellow rock past you as you ascend the mountain. They are sulfur collectors. Armed with steel bars, they work right next to the active solfatara, breaking off chunks of natural sulfur. Primitive gas masks - or, for many of them, simply a piece of wet cloth - provide poor protection against highly toxic sulfur oxide fumes. And then, once the cart or the basket is filled, they start their journey up the stony steps and down to the mining camp, halfway to the foot of the mountain, loaded with 50-70 kg of the yellow mineral. Two or three runs a day, seven days a week. After a few years of such labor, predictably, many miners develop chronic lung diseases that incapacitate or kill them. Now that Kawah Ijen is becoming a famous tourist destination, some of them are taking up new jobs as guides.
When it comes to Kawah Ijen, descriptions like "unique" or "once-in-a-lifetime" become, for a change, more than just bloated superlatives of tourist commercials. It is quite literally unique. The lake in the caldera has the greatest acidity of any natural bodies of water in the world - PH as low as 0.15 in the middle of the lake. In short, it's sulfuric acid strong enough to burn your body or eat through iron. Due to its acidic content and a significant amount of dissolved minerals, the lake has a color of lapis lazuli. The rocks surrounding it range from Technicolor yellow of pure sulfur to reddish-brown of various oxides, producing an otherworldly landscape. Again, quite literally - that's what a similar but younger, less seismically stable planet might look like.
The other unique attraction is the famous blue fire, a phenomenon that can only be observed in Kawah Ijen. Sulfuric gases, mainly hydrogen sulfide, ignite upon contact with atmospheric oxygen. The flames are invisible in daylight, but at night one can see whole areas of rock burning with bright neon-blue fire. Flowing molten sulfur keeps emitting gas, and if the weather is dry, branching streams of fire spread from the solfatara towards the lake.
The climb to Kawah Ijen caldera begins at Pos Paltuding. There's no public transport to that point - one can take a minibus from Banyuwangi in East Java as far as Licin village, then hitchhike the rest of the way. Walking from Licin would leave you winded before you even start the real hike - it's steep ascent all the way. Another alternative is to ride a motorbike, or, finally, take a local tour. Almost every guesthouse in Banyuwangi organizes those, and the program essentially consists of a rusty minivan getting you to Pos Paltuding and back.
A few years ago Kawah Ijen has been officially declared a prime tourist attraction of East Java, and an entrance fee (~8$ at the time of research) has been introduced. Officially, the visiting hours start at 01:00 AM - yes, one o'clock; it's sightseeing for the night owls. Here's the catch: to see the two unique phenomena properly, you need to be there at different times of the day. The lake looks best in the early morning, while the blue fire is only visible at night. The most rational plan is to hike up the mountain in the middle of the night and stay for the sunrise. If you take a tour, make sure it lasts long enough to experience both. Pack sufficient drinking water, as any liquid you find in Kawah Ijen is poisonous. Gas masks can be rented at the entrance down below - take one if you intend to venture close to the blue fire. And, in any case, keep a safe distance; the flames reach 600°C, and a sudden burst of sulfuric gases can cover a wide area around the solfatara, creating a threat of suffocation. So long as you stay on the trail, the risk is negligible.
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