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July 19 (UPI) — Researchers have found evidence of ancient tsunamis among the sediment layers inside a Sumatran sea cave.

The well-preserved, 5,000-year tsunami record serves as a worrisome reminder of the unpredictability of those natural disasters.

“Our geological record from a cave illustrates that we still cannot predict when the next earthquake will happen,” Benjamin Horton, a professor of marine and coastal sciences at Rutgers University, said in a news release.

Often, newly discovered geologic records can help researchers fill out the historical record of earthquakes and volcanoes in a specific region, improving prediction models. But tsunamis have so far evaded understanding.

“Tsunamis are not evenly spaced through time,” said Charles Rubin, a geologist at Singapore’s Nanyang Technological University. “Our findings present a worrying picture of highly erratic tsunami recurrence.”

The extra-long tsunami record — the first found in a sea cave and one of the most pristine in the world — proves a pair of disaster waves can be separated by several centuries or just a few decades.

The sedimentary discovery was found in a sea cave located just south of Banda Aceh, Indonesia. Layers of sand and bat droppings deposited by massive waves reveal a history of tsunamis between 7,900 and 2,900 years ago. The deadly 2004 tsunami that devastated much of Southeast Asia washed away sedimentary layers younger than 2,900 years old.

The 5,000-year sedimentary snapshot reveals the presence of 11 tsunamis generated by the region’s active fault line, stretching 3,300 miles from Myanmar to Sumatra in the Indian Ocean.

The cave was discovered by archaeologists who alerted geologists to its potential scientific value.

Researchers detailed the novel discovery in the journal Nature Communications.