Nobody had any clue. There was certainly no warning. It’s part of the picture that now points to a large underwater landslide being the cause of Saturday’s devastating tsunami in the Sunda Strait.

Of course 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.

And yet it's well known that volcanoes have the capacity to generate large waves. The mechanism as ever is the displacement of a large volume of water.

Except, unlike in a classic earthquake-driven tsunami in which the seafloor will thrust up or down, it seems an eruption event set in motion some kind of slide. It isn't clear at this stage whether part of the flank of the volcano has collapsed with material entering the sea and pushing water ahead of it, or if movement on the flank has triggered a rapid slump in sediment under the water surface.

The latter at this stage appears to be the emerging consensus, but the effect is the same - the water column is disturbed and waves propagate outwards.