With at least 34 Australians missing after catastrophic flash floods tore through Nepal and neighbouring Tibet, attention is turning to how a wall of water capable of destroying roads, bridges and entire communities could have struck with seemingly little warning.
Scientists are still piecing together exactly what happened, with early theories ranging from an earthquake and avalanche to a burst glacial lake.
But a preliminary assessment now points towards a huge section of glacier breaking away and plunging more than a kilometre into the Himalayan valley below, potentially setting off a devastating chain reaction.
What happened?
A massive torrent of water, mud and debris suddenly surged through Nepal’s mountainous Rasuwa region and areas near the border with Tibet on Wednesday.
Footage captured the fast-moving floodwaters tearing through the Himalayan valley, destroying roads, bridges, buildings and other critical infrastructure in its path.
Initial reports suggested a glacial lake may have burst in the mountains above, a phenomenon that has become an increasing concern across the Himalayas.
However, researchers examining satellite and seismic data now believe a collapsing glacier is the more likely explanation.
Satellite imagery appears to show a large section of glacier breaking away before falling about 1200 to 1300 metres into the valley below.
During that enormous fall, scientists believe the ice, rock and other debris would have broken apart before crashing into the valley.
Why did it cause such catastrophic flooding?
Scientists believe the glacier collapse may have set off a chain reaction capable of creating an enormous and sudden surge of water, according to EarthSky.
One theory is that the avalanche of ice, rock and debris temporarily blocked a river in the steep valley below, effectively creating a natural dam.
Water would then have rapidly accumulated behind that blockage until the barrier gave way, unleashing it downstream.
When did the disaster happen?
The flash flooding struck on Wednesday, August 26, with communities given little time to react to the sudden torrent.
The speed at which scientists believe the avalanche and subsequent flood unfolded could help explain why so many people were caught in the disaster zone.
Significantly, there had been relatively little rainfall before the disaster, making heavy rain an unlikely primary trigger.
Where did the flood strike?
The disaster struck the mountainous Rasuwa District north of Kathmandu and close to Nepal’s border with Tibet.
The region is characterised by steep Himalayan valleys, glaciers and rivers, creating conditions where a sudden avalanche or landslide can potentially have devastating consequences downstream.
The disaster zone is also frequented by international tourists, trekkers and pilgrims, including Australians.
At least 34 Australians are currently missing, with Foreign Minister Penny Wong confirming they had been travelling in separate pilgrimage and tour groups.
Who is investigating what caused it?
Scientists in Nepal and internationally are analysing satellite images, seismic recordings and the landscape to establish the precise sequence of events.
Nepalese Foreign Minister Shisir Khanal initially suggested an earthquake may have triggered an avalanche, which then caused the flooding.
A magnitude-4.4 seismic event was recorded around the time of the disaster.
However, analysis by seismologists suggested that reading may actually have been caused by the enormous amount of debris crashing down the mountain rather than an earthquake triggering the collapse.
Other scientists say it is too early to rule an earthquake out.
Sarah Boulton, professor of geohazards at the University of Plymouth, said the strength of the tremor meant an earthquake could still have caused a mountain slope to collapse, triggering the avalanche and subsequent flood.
“Understanding exactly how this happened will occur in the next days and weeks,” she said.
Why could climate change matter?
Scientists have previously warned that warming temperatures are making parts of the Himalayas increasingly unstable as glaciers retreat and ice melts.
That does not mean climate change has been confirmed as the cause of this particular disaster.
But researchers will examine whether warming conditions contributed to the glacier becoming unstable enough for such a large section to collapse.
For the families of the missing Australians, exactly what triggered the catastrophe remains secondary to the increasingly urgent search for their loved ones.
But establishing how a section of Himalayan glacier may have unleashed such a devastating wall of water will be critical to understanding whether similar disasters could strike again.
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