On Saturday, giant waves crashed into coastal towns on the islands of Sumatra and Java, killing at least 281 people and injuring 1,016.
The waves destroyed hundreds of buildings, sweeping away cars and uprooting trees in several popular tourist destinations including the Tanjung Lesung beach resort, west Java.
Footage shared on social media showed a large wave crashing into a tent in the resort, in which popular Indonesian rock band Seventeen was performing. Members of the band were seen being swept away as the wave destroyed the stage.
What caused the tsunami?
It is thought that volcanic activity set off undersea landslides which in turn generated the killer waves.
Everyone in the region will have been aware of Anak Krakatau, the volcano that emerged in the sea channel just less than 100 years ago. But its rumblings and eruptions have been described by local experts as relatively low-scale and semi-continuous. In other words, it's been part of the background. In August 1883, Krakatoa underwent one of the most violent volcanic eruptions in recorded history:
- Massive tsunamis with waves of up to 41m killed more than 30,000 people
- Thousands more were killed by hot ash
- The eruptions were equivalent to 200 megatons of TNT - about 13,000 times the nuclear yield of the bomb dropped on Hiroshima in 1945
- The eruptions were heard thousands of kilometres away
- World temperatures dropped by more than 1C the following year
- The volcanic island virtually disappeared
The first satellite imagery returned after the event on Saturday points strongly to a collapse in the west-southwest flank of the volcano. This would have sent millions of tonnes of rocky debris into the sea, pushing out waves in all directions.
The spokesman for the National Disaster Management Agency, Sutopo Purwo Nugroho, told a news conference that another tsunami is a possibility because of the continued volcanic eruptions of Anak Krakatau. He also put out a series of tweets explaining why there was no early warning for this tsunami. He said that Indonesia's early warning system is set up to monitor earthquakes but not undersea landslides and volcanic eruptions, which can also generate deadly waves.
Rescue efforts are being hampered by blocked roads but heavy lifting equipment is being transported to badly hit areas to help search for victims.
Coastal residents near Indonesia's Anak Krakatau volcano have been warned to keep away from beaches amid fears it could trigger a new tsunami.
Indonesia is prone to tsunamis because it lies on the Ring of Fire - the line of frequent earthquakes and volcanic eruptions that circles virtually the entire Pacific rim.
In September, more than 2,000 people died when a powerful earthquake struck just off the central Indonesian island of Sulawesi, setting off a tsunami that engulfed the coastal city of Palu.
However, tsunamis caused by volcanic activity like this are less frequent.
