The Ijen volcano complex is a group of composite volcanoes located on the border between Banyuwangi Regency and Bondowoso Regency of East Java, Indonesia.
It is inside an eponymous larger caldera Ijen, which is about 20 kilometres (12 mi) wide. The Gunung Merapi stratovolcano is the highest point of that complex.
The name "Gunung Merapi" means 'mountain of fire' in the Indonesian language; Mount Merapi in central Java and Marapi in Sumatra have the same etymology.
The lake is recognised as the largest highly acidic crater lake in the world. It is also a source for the river Banyupahit, resulting in highly acidic and metal-enriched river water which has a significant detrimental effect on the downstream river ecosystem.
The pH of the water in the lake's edges was measured to be 0.5 and in the middle of the lake 0.13 due to a high concentration of sulfuric acid.
According to en.wikipedia