This is a bit of geographic information on this unusual formation:

The Ijen volcanic complex is a collection of stratovolcanoes in East Java, containing a large cauldron-shaped “caldera” that is approximately 22 kilometers (13.6 miles) across. The highest peak belongs to the volcano Gunung Merapi, which very appropriately, if unoriginally, translates as “mountain of fire.”

This emission of thermal radiation gives lava its red hue. In this case, however, the lava is actually burning something else: pockets of sulfur, which spew out of the volcanic crevices along with the molten rock.

This is amazing and we hope you get a chance to check one of these things out.

(but only with a professional tour guide 😉