The turquoise lake in the crater of the Kawah Ijen volcano looks serene and inviting. It also happens to be the world's largest acidic lake. The water in the crater lake has a pH less than 0.3 on a scale of 0 to 14 (7 is neutral). For comparison, lemon juice has a pH of 2; battery acid has a pH of 1. That acidity affects the chemistry of nearby river ecosystems, including the river Banyupahit.