Indonesia's Anak Krakatau volcano has erupted, collapsing one of its flanks in the process, which set off a tsunami. While the full scale of the disaster will not be known for weeks or months, the death toll is already well past 200, with some areas too isolated to report. Explosions are ongoing, and something much larger may be still to come.

Tsunamis are most commonly caused by undersea earthquakes, but they can also be triggered by landslides, and it is thought the tsunami, or seismic wave, was the result of one of the volcano's flanks collapsing. The localized nature of such landslides prevented people on nearby islands getting the warning an earthquake would have given. Many of the deaths were at a rock concert held on Tanjung Lesung beach, west Java where the musicians and crowd were oblivious to the danger until the first wave struck.

The eruption destabilized the volcano's flanks, triggering underwater landslides that displaced water, unleashing the giant waves. The tephra deposits that make up the bulk of the volcano are highly unstable, so there is no reason to think something similar will not happen again as the eruptions continue.

"It looks like part of the flank of the volcano is gone and the eruption is not coming out of the top of the volcano anymore, it is very close to the water level or it is coming up through the water," Professor Heather Handley of Macquarie University told the Sydney Morning Herald.

"It looks like a lot of steam, so the magma is heating up the water and converting it as well as the actual eruption."

The head of the Indonesian National Disaster Management Agency, Sutopo Purwo Nugroho called for people to stay away from coastal areas for the immediate future.

The ash plume from the eruption has been propelled far into the stratosphere, and may impede air travel in the region. Satellite images are also showing astonishing amounts of volcanic lighting.

In 1883 the island of Krakatau experienced an eruption four times the size of the largest nuclear bomb ever detonated. The explosion was heard at least 4,800 kilometers (3.000 miles) away and more than 36,000 people were killed. Most of the island was destroyed, although a new island, dubbed Anak Krakatau (Son of Krakatau), arose 44 years later from continued volcanic activity.

Geologists have feared a repeat event for a long time. Although we are in a much better position to warn inhabitants of nearby islands about such events today, the population of the region has risen so dramatically since 1883 that the effects of an explosion of similar size could be almost unimaginable. The volcano sits between Java and Sumatra, Indonesia's two most populated islands, which have almost 200 million people living on them.

Anak Krakatau has been active since it emerged from the sea in 1927, but has been intermittently erupting since June. In November it produced some very dramatic volcano lightning, amazingly caught on camera. The current eruption has been described as the strongest in decades, but is still a pale shadow of the explosion of 1883. The fear is that this may be just a premonition of what is to come.