In the morning of December 26, 2004 a 9.3-magnitude earthquake off Indonesia's western coast generated a series of massive waves that killed more than 220,000 people across 14 countries as far apart as Indonesia, Sri Lanka and Somalia.
Among the victims were thousands of foreign tourists, the majority enjoying the Christmas period on Thailand's sun-kissed southwest coast, spreading the horrors of the disaster to homes around the world.
There were no warning systems in place and not enough time for many people to find higher ground as the towering wave hit coastal areas. Others simply stared in awe and curiosity as the sea at first retreated, before rushing back as a wall of churning water.
"When you forget, you don't prepare," said Margareta Wahlstroem, head of the UN Office for Disaster Risk Reduction, who played a leading role in organising the UN response and recovery efforts a decade ago.
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"Disaster amnesia" threatens to lower defences, Wahlstroem told AFP.
"You relax, and that's dangerous... One of the big challenges in reducing disaster-risk is to keep alive this understanding."
It took around 20 minutes after the quake for the first waves -- some more than 35 metres high -- to hit the coast of Aceh, where the vast majority of Indonesia's 170,000 victims perished.
"We were flying blind, without any kind of sensors in the Indian Ocean," Charles McCreery, director of the US government's Pacific Tsunami Warning Center, told a recent conference in Jakarta.
After "100 years of calm" there have been six quakes of 7.9 or above in the Indian Ocean since 2004 in a period of "heightened activity", according to McCreery.
To prevent avoidable losses again, the Indian Ocean Tsunami Warning System -- spanning the ocean and monitored by hubs in Indonesia, Australia and India -- began operations in 2011.