Have you heard about the Blue Fire in Kawah (volcano) Ijen?
Kawah Ijen is located in East Java, near the city of Banyuwangi, it takes you 1 hour drive after crossing the Bali Strait between Bali and Java.
You will drive early in the morning to the basement and trek for 2 hours (depending on your fitness condition). After reaching the top, you can descend into the crater to see the blue lights. On the way, you will encounter the workers extracting the sulfur, some of them will labor it and sell it under different shapes.