The beauty of the Hawaiian Islands is stunning, but it comes at a cost. The chain of islands sits precariously in the middle of the Pacific Ocean, leaving its inhabitants vulnerable to tsunamis generated by both nearby and distant earthquakes.

One such tsunami occurred on April 1, 1946, after an 8.1-magnitude earthquake in the Aleutian Islands. The tsunami that hit Hawaii measured up to 10 feet tall, claimed roughly 170 human lives, and caused over 10 million dollars in damages.

Researchers recently uncovered a large mass of marine debris in a Makauwahi sinkhole on the island of Kauai that indicates that the islands were nailed by a “monster tsunami” some 500 years ago. The tsunami, estimated to have occurred between 1425 and 1665, was generated by a 9.0-magnitude earthquake near what is now Alaska.

The mouth of the sinkhole sits 23 feet above sea level and 328 feet inland, indicating that the tsunami had the size to throw the debris that high and that far from the shore. Radiocarbon dating of the debris was used to estimate its age.

“You’re going to have great earthquakes on planet Earth, and you’re going to have great tsunamis,” said lead author Rhett Butler, a geophysicist at the University of Hawaii at Manoa. “People have to at least appreciate that the possibility is there.”

The researchers caution that Hawaiian residents should be prepared for a similar monster tsunami to hit in the near future. The state government created evacuation maps informed by the tsunami of 1946 as well as the Tohoku tsunami that hit Japan in 2011.

“[The Japan earthquake] was bigger than almost any seismologist thought possible,” said Butler. “Seeing the devastation it caused, I began to wonder, did we get it right in Hawaii? Are our evacuation zones the correct size?”

The report of the sinkhole debris discovery was published in the October 3 issue of Geophysical Research Letters.