People gathered in prayer and visited mass graves in Indonesia’s Aceh province on Thursday to mark 20 years since the massive Indian Ocean tsunami hit the region in one of modern history’s worst natural disasters.

Understanding of the Sumatran subduction zone and its hazards has increased since the 2004 Indian Ocean earthquake and tsunami.

Christmas in 2004, a 9.1 magnitude earthquake triggered tsunami waves that killed some 230,000 people across a dozen countries.