A Lake That Kills
Mount Kelut, a 1,731-meter-high stratovolcano in East Java, holds a deadly feature in its summit crater: a lake. This lake becomes a catastrophic liability during an eruption. On May 19, 1919, one of the deadliest eruptions of the 20th century provided a horrific demonstration. The eruption violently ejected the lake's contents—an estimated 38 million cubic meters of water—which mixed with volcanic ash and debris. This created immense, fast-moving mudflows known as lahars.
These lahars raced down the volcano's flanks at speeds up to 60 km/h, traveling nearly 40 kilometers from the summit. They inundated over 100 villages, destroying everything in their path and turning the town of Blitar into a "sea of lahar". In less than an hour, more than 5,100 people were killed. The disaster was so swift and destructive that it prompted the Dutch colonial authorities to seek a permanent engineering solution, leading to the establishment of Indonesia's first volcanological authority.
Engineering a Volcano
Previous attempts to mitigate Kelut's lahars, like a dyke built in 1905, had been completely overwhelmed. After the 1919 disaster, engineers abandoned the idea of controlling the mudflows and instead focused on preventing their formation by draining the source: the crater lake. The ambitious project began shortly after the eruption and took until 1926 to complete. Workers constructed a system of seven drainage tunnels through the side of the volcano. This initial effort successfully lowered the lake level by 50 meters and reduced its volume from around 40 million cubic meters to less than 2 million.
The tunnels proved their worth during the 1951 eruption. With a much smaller volume of water in the crater, the resulting lahars were minimal and no lives were lost. The eruption, however, damaged the original tunnels and deepened the crater floor by 70 meters. After another deadly lahar in 1966 killed over 200 people, a new, deeper tunnel called the Ampera Tunnel was excavated in 1967 to once again lower the water level. This system of tunnels is one of the world's first and most enduring examples of active volcanic mitigation engineering, a constant battle against the mountain's geology. Subsequent eruptions and the growth of a new lava dome in 2007 have required further modifications, showing the continuous need to manage the water in Kelut's crater.