Coming up from a dive on many a site in Indonesia and one thing you will notice are volcanoes. The country is one of the most geologically active in the world - in fact, much of the Indonesian archipelago could be described quite simply as one big chain of volcanoes, some still active.

Probably the most well known of Indonesia's volcanoes is Krakatoa, famously misidentified as being 'East of Java' in the 1969 film title, but in reality lying west in the strait that separates Java from Sumatra.  Krakatoa erupted catastrophically on August 27, 1883, almost completely destroying the island and killing anywhere between 35,000 and 100,000 people with a deadly combination of pyroclastic flows, ash falls and tsunamis.  The pressure wave from the final, cataclysmic explosion was recorded around the world and the huge volume of ash propelled into the atmosphere caused average global temperatures to fall by 1.2 degrees Celsius in the year following eruption.

A yacht moored in peaceful Jemeluk Bay in Amed, with Mount Agung - Bali's tallest volcano - looming in the background.
A yacht moored in peaceful Jemeluk Bay in Amed, with Mount Agung - Bali's tallest volcano - looming in the background.

However, the 1883 eruption of Krakatoa was merely a hiccup compared to some of Indonesia's more spectacular eruptions in the past.  In 1815 Mt Tambora in Sumbawa erupted with a Volcanic Explosivity Index of seven - four times the strength of Krakatoa. An estimated 100 cubic kilometres of pyroclastic material was ejected during the eruption and the mountain went from being a respectable 4,300m in height, to only 2,851m.  The year after the eruption became known as the 'Year Without a Summer' in countries as far away as America and the atmospheric conditions and spectacular sunsets inspired famous landscape artists such as J. M. W. Turner.

Cows taking a dip at sunset at the foot of Sangeang's volcano in Sumbawa.
Cows taking a dip at sunset at the foot of Sangeang's volcano in Sumbawa.

The prize for the most spectacular eruption in recent history goes to the Toba event, which occurred at what is now Lake Toba in Sumatra between 67,000 and 75,000 years ago.  The eruption had an estimated VEI of 8 - described as mega-colossal - and is possibly the largest explosive volcanic event within the last 25 million years. The explosion caused such changes to the global climate that scientists believe a planet-wide die-off of many species occurred as a result. And it has been suggested that the entire human population was reduced to just a few tens of thousands of individuals by the Toba event, a genetic 'bottleneck' that has influenced our genetic make-up ever since.

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1. Pulau Sangeang, Sumbawa

Sangeang has two main cones, Doro Api and Doro Mantoi, and is a very popular muck-diving stop for liveaboards running from Bali to Komodo and back. Sangeang last erupted in 2014, sending a huge cloud of ash into the atmosphere and disrupting flights between Australia and Bali.

2. Mount Batu Tara, Pulau Komba, East Nusa Tenggara

This volcanic island in the Flores Sea spits glowing rocks and ash on a regular basis and is a common overnight stop for liveaboards. After their evening meal, guests can sit out on deck and watch the eerie orange glow from one of Indonesia’s most active volcanoes.

3. Pulau Weh, Sumatra

Although Weh is no longer considered an active volcano, the island has one site where divers can explore a series of bubbling vents on the sea floor in just 10-15m. The volcanic original of the bubbles is obvious from the strong, sulphurous smell that fills the air in the small bay where the vents are located.

4. Banda Api, Banda, Maluku

The active volcano known as Banda Api is part of a group of islands in the northern section of the Banda Sea. Api last erupted in 1988, sending a flow of lava down its flanks and into the sea, demolishing reefs in the process. However, the corals have bounced back and the dive site named ‘Lava Flow’ is now an amazing field of hard corals.

5. Mahengetang, Sangihe Talaud, Sulawesi

Due north of Sulawesi, a chain of small islands extends the ‘Ring of Fire’ into the Philippines. At Mahengetang, a submerged volcanic seamount called Banua Wuhu reaches to just meters beneath the surface of the sea, giving divers a unique opportunity to explore the bubbling boulders that make up the tip of the volcano.

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