The last widely devastating tsunami in the Indian Ocean was that ... as close as it was off Sumatra. Before installing a warning system, experts need to identify and map areas of risk.

Survivors and victims' relatives will this week mark the 20th anniversary of the 2004 Indian Ocean earthquake and tsunami, which killed more than 220,000 people across more than a dozen countries.

In 2004, a 9.1 magnitude earthquake off the island of Sumatra in western Indonesia triggered a tsunami in the Indian Ocean that killed more than 220,000 people across 12 countries. Ali Rogin ...