Ijen Crater (Kawah Ijen) is the world's largest acidic lake. It is located in East Java of Indonesia just next to Bali.
Great adventurous experience you would get while hiking Ijen. Ijen is an active volcano with high amount of Sulfur. You will find local miners struggle their live uphill and down carrying very heavy Sulfur to be sold disproportionately. Stay in a camp nearby the crater or start hike in the dawn could be the best option. Trek down to the edge of crater and you will find amazing phenomenon of blue fire, it comes up from the Sulfur. Don't forget to bring proper mask because high Sulfur concentration in the air.
Be caution not to swim or even touch the lake water because it is very acid. Nevertheless hike to the top of the crater rim and you'll see scenic view of the volcano, combination of yellow sulfur, turquoise lake and blue sky if you get lucky.