An active volcano in Indonesia has been described as one of the world’s strangest volcanoes, as its activity produces blue flames instead of red lava.

Olivier Grunewald, a photographer based in Paris explained the phenomenon of Kawah Ijen volcano located in Banywang Regency in Java. According to Grunewald, the blue glow in the volcano is not lava, but the light turns that colour because of the combustion of sulphuric gases, the Express reported.

The gases leak through cracks in the volcano at high temperatures and pressure then ignite when they contact with air.

Grunewald said that some of the gases cool down a little and condense into liquid sulfur, which continues to burn while flowing down the slope of the volcano.

Blue flames can also be seen at the base of the volcano’s plume when it erupts and ash explosions happen. Despite occurring continuously, the blue burning can only be seen at night.

The unusual phenomenon has been reported in a number of documentaries over the years, which has boosted the number of tourists who visit the volcano.

The volcano also has miners who work in the volcano. According to the Smithsonian.com, the miners extract sulfuric rocks for their use in the food and chemical industries.

Grunewald said most of the miners at the volcano do not have gas masks, which makes them vulnerable to health problems due to consistent exposure to toxic gases.