ALI ROGIN: Marks 20 years since one of the worst natural disasters in modern history.

In 2004, a 9.1 magnitude earthquake off the island of Sumatra in western Indonesia triggered a tsunami in the Indian Ocean that killed around 230,000 people across 12 countries.

ALI ROGIN (voice-over): The wall of water swept up nearly everything in its path, cars, buildings and people.

Six-year-old Siddiq was one of them.

The wave ripped him from his mother sow does arms.

His body has never been recovered and she's found it impossible to move on.

SAUDAH, Tsunami Survivor (through translator): If Siddiq is still alive and I'm still alive, I want him to come home and for us to see each other before I die.

But if he is gone, I wholeheartedly accept that is God's will.

But I still believe he is alive.

ALI ROGIN (voice-over): Outside her home in Banda Aceh, Indonesia, neighborhood life goes on around the wreckage of two Coast Guard ships that washed up 20 years ago.

A daily reminder to residents of the disaster.

Saudah's story is not unique.

In communities across the Indian Ocean, the devastation was unlike anything the world had seen.

As the weeks wore on and the search for survivors dwindled, the loss of life ticked upwards until the death toll ultimately stood near 230,000.

At memorial sites for the victims around the region this week, survivors and loved ones marked the anniversary.

URAI SIRISUK (through translator): I felt that the waves took my daughter away.

I was so mad at the sea.

I cannot bring myself near it and not even have my feet in the sand.

I wouldn't come around here if necessary.

Never.

The sea took my daughter from me.

ALI ROGIN (voice-over): And in Banda Aceh, the sea produced waves of up to 100 feet.

Yet there were no warning sirens, resulting in the deaths of nearly 130,000 people here.

Now the city has a new quake detection and tsunami warning system, which its residents hope will remain ready but unused.

ALI ROGIN: Earlier this week, I spoke with Daniel Bogado, the director and executive producer of a new documentary called "Tsunami: Race Against Time."

Why is it important to look back on this tsunami 20 years on?

DANIEL BOGADO, Executive Producer, "Tsunami: Race Against Time: Well, I think it's exactly, precisely because of the amount of time that has passed.

20 years is a long period of time.

So it means there's a whole generation of adults who have no living memory of the tsunami.

A lot of people learned a lot of information about what tsunamis are.

What are the warning signs from the event itself.

We wanted to create a historical document and also memorial for the lives that were lost.

But it also serves a bit of an educational purpose.

ALI ROGIN: You uncovered an enormous amount of video and tracked down many survivors, got some really harrowing recollections.

One really struck us.

This is a man describing how he got hold of his brother and they ran away as a wall of approach.

MAN: Them and everyone was rushing in the other direction away from the sea.

And Theo and I just joined the rush.

As were running, were holding hands.

I do recall Theo saying, what about Mum and Dad?

And I didn't have answer, really.

ALI ROGIN: What did survivors tell you that surprised you the most?

DANIEL BOGADO: You know, I think a lot of us have a lot of wrong notions about what a tsunami looks like.

You know, in the films, it looks like this giant wave, and it's actually much more like a wall of water.

And it's misleading.

It's so misleading that a lot of people would see the water go back and they would just stand, curiously looking, not realizing the danger they were in.

And so a lot of the contributors have absolutely harrowing stories that come from that, not knowing exactly what was happening.

Even once the first wave came, often there was a second wave or a third wave, or even a fourth wave in which people were caught up because they didn't know that's what happens.

And so, from the perspective of survivors, when you go minute by minute, their experience, it's almost like a horror film, because you don't really understand what's happening.

And it's all incredibly unpredictable.

ALI ROGIN: Many of the people describe what it was like to be hit by the tsunami, including this woman who described the sheer force of it.

WOMAN: I was underwater and I had no air.

I remember the feeling of the wave passing on my face and trying to tear up my nostrils and opening my eyelids like immense force.

ALI ROGIN: How have people coped with the trauma of this 20 years on?

DANIEL BOGADO: Oh, it's different for everybody.

I did a series on September 11, and it was a similar situation that we're approaching people 20 years later, you track them down and you're a bit afraid that when you call them, they'll be angry at you for bringing up the worst day of their life.

But actually that I don't think that ever happened.

People were just incredibly gracious, incredibly kind, and not in every case, but in most cases, very willing to share their story.

They felt that something powerful had happened to them.

There was a certain level of wisdom about the things they experienced, the things they saw with their own eyes.

ALI ROGIN: How much better are we equipped to predict a natural disaster like this one now?

DANIEL BOGADO: With tsunamis in particular, we're much better equipped than were 20 years ago.

Part of the problem was that there was no tsunami warning center in the Indian Ocean.

And the series follows a group of scientists who realize that there's been a massive earthquake, that there's a very likely tsunami, but they don't have anybody they can contact any to warn, to evacuate the beaches.

So all of that now has changed.

Now there's tsunami warning centers in every major ocean basin.

We were able to get word out much, much quicker than were able 20 years ago.

But I think the lesson of the tsunami really is not just about the tsunami, for disasters in general or things that people might see are on the horizon.

We always wait for it to happen before we respond.

That's the real lesson of the tsunami, that we should be thinking forward about the challenges that we have and do things before thousands of lives are lost or afterwards.

ALI ROGIN: You've talked to so many people who have been through unimaginable trauma, not just after the tsunami, but other worldwide tragedies as well.

What have these interviews, particularly as it relates to the tsunami, taught you about the human condition?

DANIEL BOGADO: When people get very close to this line separating life from death, people are always thinking about very similar things.

They're thinking about their loved ones, they're thinking about getting home to their family.

Wherever you look after that, you see people coming together to want to help each other.

And you saw that on tsunami in every country, you know.

And so I think it does say something about the human condition that, you know, we are social animals.

We do want to help.

We do want to do good.

But it's actually one of these ironies that it's this incredible, unique situations, these huge tragedies where they really show these, you know, brighter side of the human condition.

ALI ROGIN: Daniel Bogado, whose new documentary is "Tsunami: Race Against Time."

Thank you so much for joining us.

DANIEL BOGADO: Thank you very much for having me.