Another tsunami could strike Indonesia, experts have warned, after a powerful wave caused by a volcanic eruption killed hundreds when it swallowed coastal settlements, taking earthquake-focused disaster monitors by surprise.
While tsunamis are often triggered by earthquakes, in this case experts believe the deadly waves were generated by an eruption of the Anak (or "child of") Krakatoa volcano, which could have caused a large undersea landslide or flow of molten rock into the water.
The tsunami "appears to have been caused by an underwater collapse" of part of the volcano, said David Rothery, a professor of planetary geosciences at Britain's Open University.
Anak Krakatoa is an island that emerged around 1928 in the crater left by Krakatoa, whose massive 1883 eruption killed at least 36,000 people.
The tsunami that struck on Saturday was the third natural disaster to hit Indonesia in six months.
The country has 127 active volcanoes and lies on the Pacific Ocean's "Ring of Fire" where earthquakes and volcanic eruptions are frequent.
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Anak Krakatoa, located in the Sunda Strait between Java and Sumatra islands, is close to densely populated areas.
The volcano has been particularly active since June, noted Jacques-Marie Bardintzeff at the University of Paris-South.
"We were helpless given how sudden" the event took place, Bardintzeff said.
"The time between cause and effect was a few dozen minutes, which was too short to warn the population."