Anak Krakatau on 5 August 2018 (top) and on 11 January 2019 showing the edifice after the collapse event (bottom). The two drone photographs show approximately the same area.
Photos courtesy of Øystein Lund Andersen (top) and James Reynolds (bottom).
From Smithsonian Institution Global Volcanism Program.

Anak Krakatau Volcano, Sunda Strait, Indonesia

On 22 December 2018 at approximately 20:55 local time (WIB, UTC+7), a period of moderate eruptions of the Anak Krakatau volcano that began in June 2018 culminated in a massive flank failure and submarine landslide. The event produced tsunami waves that reached an elevation of 85 m at neighboring Rakata Island, 4 km away, and hit the populated coasts of northwest Java and southern Sumatra 30-60 minutes later where 437 people were killed. Ultimately 2 cubic km (70 billion cubic feet) of material was lost from the island, collapsing the 328 m volcanic cone into a crater opening into the ocean and decreasing the height of the island by 200 m.

The toll on human life was exacerbated by the timing (at night during a holiday week when tourist destinations were full). In addition, because of the nature of the tsunami trigger – an extended period of eruptions without a large seismic event – residents reported no warning signs such as receding water or earth shaking before the waves came ashore.

It appears that a perfect storm of magma-tectonic processes at Anak Krakatau culminated in the 22 December 2018 tsunami disaster. Leading up to the event, different sensors, and methods measured distinct anomalous behaviors, which in hindsight can be deemed precursory. However, at the time and when considered individually, none of the parameters, including the thermal anomalies, flank motion, anomalous degassing, seismicity, and infrasound data, were sufficiently conclusive to shed light on the events that were about to unfold. – Walter et al., (2019) Complex hazard cascade culminating in the Anak Krakatau sector collapse.