Tsunamis are often triggered by volcanic eruptions, earthquakes, landslides, or meteorites—and the waves can spread out thousands of miles. | National Geographic

On 26 December 2004, a deadly tsunami hit the Indian Ocean, affecting coastal communities in more than a dozen countries. India’s Andaman and Nicobar Islands were one of the affected areas. For the BBC’s Geeta Pandey, what began as a vacation turned into an unforgettable experience covering one of Asia’s biggest disasters. Twenty years later, she recounts what she saw. | BBC News India

In 2004, a large earthquake in the Indian Ocean triggered the largest tsunami in recorded history. One of the countries hit by the tsunami was Sri Lanka, where the wave caused one of the worst train disasters in human history when it hit the Queen of the Sea passenger line, which was carrying 1,500 passengers along Sri Lanka's west coast. Watch Destruction Decoded on Wednesdays at 8pm on National Geographic UK. | National Geographic UK