The Kawah Ijen volcano in East Java, Indonesia, is home to the Blue Fire Crater, a destination famous for its luminous blue lava flows.

But that's not lava. It's flame. The phenomenon is caused by molten sulfur and flaming sulfuric gases, which are released by fumaroles near the base of the volcano's crater. Photographer Reuben Wu recently made the almost three-hour trek—that's two hours up the volcano rim, and another 45 minutes down to the bank—in order to snap the faux lava flows in the dark of night.

The high pressure and temperatures, which can reach 1112 degrees Fahrenheit, often lead to flares that erupt as high as 16 feet. And at the top of the volcano is Kawah Ijen Crater Lake, the largest acidic lake world, filled with hydrochloric acid that turns the water green. 

You can check out more of Wu’s photos on Instagram and Facebook, and see the incredible blue flames in action in the video below.

[h/t Colossal]