he volcanic cone of Ijen dominates the landscape at the eastern end of Java. Crater of Ijen is filled by a spectacular turquoise blue lake, its surface streaked in wind-blown patterns of yellow sulphur.
Kawah Ijen is the world’s largest highly acidic lake and is the site of a labor-intensive sulfur mining operation in which sulfur-laden baskets are hand-carried from the crater floor.
Many other post-caldera cones and craters are located within the caldera or along its rim. The largest concentration of post-caldera cones forms an E-W-trending zone across the southern side of the caldera. Coffee plantations cover much of the Ijen caldera floor, and tourists are drawn to its waterfalls, hot springs, and dramatic volcanic scenery.
Quoted From http://www.eastjava.com/books/ijen/main.html