Miners tread down into the crater to collect blocks of bright-yellow sulfur, which forms when gaseous and molten sulfur emerges from fumaroles -- volcanic vents -- and hardens.

As soon as the sulfur is exposed to the ambient air temperature, it hardens into the more-familiar bright-yellow solid. The odor is terrible -- like a combination of rotten eggs and ammonia.

A fresh supply of yellow froth (probably the same type of elemental sulfur found in the "cool acid-brine" crater lake at Kawa Ijen) is brought up by the plume and pushed out toward the crater wall.