Scientists have found evidence of a massive tsunami that devastated Hawaii some five centuries ago, prompting Aloha State officials to greatly expand their tsunami evacuation plans. About 500 years ago, a powerful Alaska earthquake sent towering waves up to 30 feet (9 meters) tall crashing down on Kauai, leaving behind fragments of coral, mollusk shells and coarse beach sand in a sinkhole.

Evidence of this colossal tsunami surfaced in the late 1990s during the excavation of the Makauwahi sinkhole, a collapsed limestone cave on the south coast of Kauai. About 6.5 feet (2 meters) below the surface researchers found a bounty of old debris that must have come from the ocean.

The quake, likely a magnitude 9.0, sent the mighty waves toward Hawaii sometime between 1425 and 1665, the study found. It’s very possible that another large Alaskan earthquake could trigger a similar tsunami in Hawaii’s future.

This massive tsunami was at least three times the size of the damaging 1946 tsunami, which was driven by an 8.6-magnitude earthquake off the Aleutian Islands. Mammoth tsunamis, like the one described in the study, are rare, and likely happen once every thousand years. There’s a 0.1 percent chance it could happen in any given year.

Results of this study have already prompted Honolulu officials to modify their tsunami evacuation maps. The new maps, which will affect nearly 1 million people who live in Honolulu County, would include more than twice the area of evacuation in some areas. County officials hope to distribute the new maps by the end of 2014.