One of the most unusual volcanoes on the world is Kawah Ijen. It contains the largest lake of acid in the world but especially impressive are the eerie blue flames around the solfataras in the crater – an unforgettable sight in the night.


There is Ijen and Kawah Ijen. The first – Ijen (also Kendeng) – is the whole complex of volcanoes in this area, spread around the giant, approximately 20 km wide caldera. Highest volcano here is the 2,799 m tall Gunung Merapi. But the story goes about another part of Ijen: a crater which is located to the west from the mighty Gunung Merapi – Kawah Ijen (Ijen crater).

The lake of acid in Ijen Crater, Indonesia
The lake of acid in Ijen Crater / © CEphoto, Uwe Aranas, Wikimedia Commons

This crater has several very unusual characteristics, including an enormous lake of pure acid and… cracks which emit burning gas which in the night glow in eerie blue color. But, when the day comes, group of men arrives to hack off large chunks of pure, yellow sulphur which constantly is emanating from the ground.

Lake of acid

When looking on the satellite image, Kawah Ijen attracts the attention with a lake in unnatural, turquoise color. This crater lake is filled with almost pure sulphuric acid and is the largest highly acidic lake in the world. Lake is some 0.7 by 0.9 km large, it contains some 30 million m³ of acidic liquid (hyperacidic brine) and is some 200 m deep.

The lake of acid in Ijen Crater, Indonesia
The lake of acid in Ijen Crater / Berry, Flickr / CC BY-SA 2.0

Adventurer George Kourounis in 2008 in a rubber boat measured the acidity of the lake: at the edges the acidity was 0.5 pH and in the middle – 0.13 pH. Thus it is a lot more acidic than the acid in car batteries.

From the lake starts a river – Banyu Pahit. Of course, this also is very unusual river with highly acidic water. In this water have been dissolved metals – thus it is natural source of chemical pollution.

In a case of powerful eruption the wave of acid from the lake would create a potential danger to surrounding villages.

Sulphur mining

At the south-eastern bank of the lake are located fumaroles which emit sulfurous gases. Over the time here have formed rich deposits of this yellow substance.

Sulphur miner in Ijen Crater, Indonesia
Sulphur miner in Ijen Crater / Jean-Marie Hullot, Wikimedia Commons / CC BY-SA 2.0

People have installed a system of ceramic pipes and molten sulphur flows through these pipes and accumulates in heaps on the ground.

Some 200 local miners are mining the sulphur – some 14 tons per day. This extremely hard work is done by the hand tools. Chunks of sulphur are carried away on the back. Each miner carries some 70 – 90 kg of sulphur several times per day. The work is hazardous for the health, especially for the respiratory system and teeth – but working with gas masks is not easy either, thus workers often prefer to work without them.

Blue fire

The sulphurous gas and condensed liquid in some locations is burning. During the day time flames are not that bright but in the night the sight is unforgettable – the flames have deep blue color. For most part the flames are ignited by people – by torches of sulphur miners who often work also during the night time.

The blue fire of Kawah Ijen, Indonesia
The blue fire of Kawah Ijen / Dodi Mulyana, Flickr / CC BY 2.0

These flames are up to 5 m high and the temperature of the flame reaches up to 600° C. Part of the gas is condensed as a liquid which continues to burn.

Some other volcanoes around the world also have such blue fires but the Blue Fire of Ijen is by far the largest in the world.

Nighttime ascent on this mountain is increasingly popular tourist attraction and few regret the effort – the sight is truly unforgettable. Though one should be very careful here because this is very hostile environment for any forms of life. Of course, assistance of local guide is of high value, thus also giving some input into economy of the hospitable, hard working local people.


  1. Global Volcanism Program, Ijen. Accessed on February 4, 2017
Kawah Ijen on the map
Travelers' Map is loading...
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Location, GPS coordinates: 8.0581 S 114.2424 E
Categories: Volcanoes, Lakes and streams
Values: Geology, Visual
Rating: 4 out of 10 stars
Where is located? Asia, Indonesia, East Java, Banyuwangi and Bondowoso Regencies
Height: 2,154 m (level of crater lake)
Alternate names: Idjen, Kawah Ijen (Ijen crater)

Landmarks of Indonesia

Kelimutu crater lakes, Flores
Kelimutu crater lakes, Flores / H. Hansson, Wikimedia Commons / CC-BY-SA-3.0
Indonesia is a true land of wonders. There are few other countries in the world which can offer similar array of diverse, unique and beautiful natural and cultural monuments.


Mount Cleveland, Aleut Islands
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This category includes most unusual and interesting volcanoes of the world.

Over the last 10,000 years in some 1,500 places around the Earth through the crust of the planet has been emitted lava, ash and gases from the mantle of Earth. Each of these places could be considered to be an active volcano. Every year some 50 – 70 volcanoes are erupting, at any moment there are some 20 – 30 eruptions on-going.

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