A large part of the south-western cone of Krakatau appears to have given way. This and probable collapses undersea have caused a massive displacement of water.
One theory suggests this resulted from recent lava flows combining with accumulated material over many decades. This created a tipping point and ultimately caused the collapse. This may have exposed the conduit (the plumbing of the volcano), allowing sea water to enter and therefore causing the huge eruptive plume.
Many readers have asked to see what a large eruption could be like – we filmed this back in October.