Ijen's not just one, or two, but a whole strip of volcanic peaks that carves through the lands of Java, just shy of the Bali Sea. Thanks to its sinewy ridges of ochre-hued stone and breathtaking vistas of smoke-belching craters, it's risen to become one of the top trekking destinations in all of Indonesia.

Travellers can scramble up steep slopes, through sporadic pockets of jungle, along rock-ribbed trails to reach the point where the mountain levels out. The prize is a much-photographed turquoise crater lake, spreading between the chiselled slopes of black rock and stone, like a mirror wedged into an active volcano.

There's also one final treasure for Ijen tour guides to reveal: The so-called Blue Flame. It's an otherworldly sight, the product of sulfuric gases rising and meeting the atmosphere where they combust in an instant. It can be viewed on nighttime treks to the edge of the crater; zig-zag down challenging trekking paths before seeing the curious plumes of cobalt fire erupting from the mountainside.