Sediments deposited in a coastal cave in Sumatra are a record of giant Indian Ocean tsunamis occurring over thousands of years, scientists say. The limestone Acehnese cave on Sumatra's northwest coast is a geological record of the frequency of earthquake-induced giant waves like the catastrophic tsunami of December 26, 2004, they said. Scientist said they can reveal that record by analyzing the cave's tsunami-borne sediments, piled up in layers over centuries whenever waves large enough to breach the cave's entrance carried sand and debris inside. The tsunami layers are easy to see between layers of bat droppings, they said. "The tsunami sands just jump right out at you because they're separated by guano layers," researcher Jessica Pilarczyk told the BBC. "There's no confusing the stratigraphy (layering)." Pilarczyk is part of a team of researchers from the Earth Observatory of Singapore, a part of Nanyang Technological University investigating the coastal history of Indonesia's largest island. The cave lies about 100 yards from the current high-tide level and its raised entrance prevents sea water from flooding into the opening, except when there is a tsunamis or severe storm surge, the researchers said. So far the sediments have revealed the effect of between seven to 10 tsunamis from about 7,500 to 3,000 years ago, they said. "This coastal cave is a unique 'depot center,' and it's giving us a remarkable snapshot of several thousands of years, allowing us to figure out every single tsunami that would have taken place during that time," Pilarczyk said.