Scientists used sonar equipment to image the giant chunks of rock that slid into the ocean when one side of Anak Krakatau collapsed, the BBC reported.
Some of these blocks are 70-90m high.
Their plunge into the water produced tall waves that tore across the shorelines of Java and Sumatra on December 22, 2018.
Over 400 people around the Sunda Strait died in the night-time disaster, and thousands more were injured or displaced.
Researchers have been trying to reconstruct what happened ever since. But all their studies to date have been based on what can be seen above the water.
Professor Dave Tappin and colleagues realised they had to investigate the island volcano's missing mass - now under the ocean's surface - or they would never truly get a full description of Anak Krakatau's failure.
A multibeam echosounder was brought in to map the seabed.
"Early models of the collapse were based on satellite imagery that only looked at the subaerial parts of the volcano," the British Geological Survey scientist told BBC News.
"Our bathymetry is imaging at 200 mere water depths and we are seeing triangular-shaped blocks, which are basically coherent and they formed, before the collapse, the southwestern flank of Anak Krakatau."
On the islands in the immediate vicinity of Anak Krakatau, trees up to 80 metres above the normal sea surface were torn from their roots.
( With inputs from IANS )