Kawah Ijen is the world's largest highly acidic crater lake — a one-kilometre-wide body of intensely turquoise water that fills the crater of an active volcano at 2,148 metres above sea level. The colour comes from dissolved sulphuric compounds; the pH is below 0.3, comparable to battery acid. The lake is beautiful and terrifying in roughly equal measure.
The crater can be reached by hiking down from the rim — a steep 300-metre descent on a rough path used daily by the sulfur miners. At the lakeshore, sulphuric fumes from the active vents are overwhelming without a gas mask; even with one, extended exposure is ill-advised. But the view from the rim looking down into this chemical cauldron, with steam rising and the water glowing electric blue-green, is one of the most otherworldly natural sights in Indonesia. The hike to the rim alone takes 90 minutes from the Paltuding basecamp; allow another hour for the crater descent and ascent if you want to reach the lakeshore.