When a devastating tsunami struck several countries in 2004, it killed over 220,000 people and caused damage that would take years to fully repair.
Indonesia, Sri Lanka, India, and Thailand bore the brunt of the tsunami, which began due to the movement of volcanic plates beneath the surface of the sea.
20 miles beneath the ocean floor, the India plate and the Burma microplate subducted, which is where the plates collide and one slides under the other.
That led to an 800-mile rupture, which triggered a massive earthquake that reached a magnitude of 9.1 on the Richter scale.
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The movement of the plates also caused the ocean floor to rise by 40 metres, which in turn triggered the tsunami.
As giant waves of destruction headed for the coastlines of multiple countries, some people captured the early stages of the disaster through holiday footage.
Among the footage is a video showing the frightening power of a tsunami, as people watching it could see the waves were going backwards.
Taken by YouTuber Pilot90, it shows one of the later waves that followed the initial hit of the tsunami, andyou can see the waves moving in reverse.
The reason this happens, according to the National Oceanography Centre, is that a tsunami will draw water back from the beach to 'feed' the gargantuan wave.
What you're seeing is the tide going out with alarming speed and power to feed the next tsunami wave.
Viewers have been stunned and scared by seeing the waves moving in reverse, admitting it was a 'weird and scary' sight to see the ocean not behaving how it ought to.
While some were saying how the waves going backwards 'freaked them out' thanks to the 'power and energy underneath the surface of the water', others pointed out that this was a major warning sign of a tsunami.
They explained that when it looks like the waves are moving in reverse and the water is drained off the beach, it means a tsunami is on the way, as all that water will return with terrifying force.
If you ever see such a thing, then 'you should be running now', according to their advice, and try to look for 'higher ground'.
A survivor of the Boxing Day tsunami of 2004 recalled being swept away from his family, becoming separated from his brother and never seeing his parents again after that.
After the devastation, people from around the world donated billions to help pay for the long repair works.
Topics: Weather, World News, Science, YouTube
Joe Harker