A limestone cave discovered in Sumatra, near the source of the deadly tsunami that hit Indonesia on 26 th December, 2004, contains the footprints of past ancient tsunamis dating back 7,500 years. The discovery provides the most detailed timeline for tsunamis that have occurred in the region and could help scientists better understand the nature and frequency of these destructive waves.

The cave is located within a couple of hundred metres off the coast near Banda Aceh and is protected from storms and winds. Only massive waves that bombard coastal areas are able to reach the cave. When such a wave occurred, it brought with it sand deposits that became layered between bat droppings over thousands of years, forming a rare natural record of tsunamis.

While the cave was discovered in 2011, researchers have only just completed radiocarbon analyses of materials, including clamshells and the remains of microscopic organisms. Their results showed that there have been 12 tsunamis within the last 7,500 years, including the wave that struck in 2004. Before then, the last once occurred about 2,800 years ago, but there were four others in the preceding 500 years.

Researchers are now trying to determine the size of the tsunamis that entered the cave. "By learning about the type of tsunamis that happened in the past, maybe we can do planning for mitigation for the next tsunami," said Nazli Ismail, head of the physics and geophysics department at Syiah Kuala University in Banda Aceh who worked on the project.

Indonesia is an archipelago located on the so-called "Ring of Fire," a horseshoe of fault lines and volcanoes surrounding the Pacific Basin. It is home to some of the world's biggest and deadliest seismic activity.

By April Holloway