The largest tsunami in Hawaii's recorded history struck in 1946. Now, scientists have uncovered evidence that they say indicates the 8-foot wall of water was dwarfed by one as much as 30 feet high that came centuries before. The discovery of the Makauwahi sinkhole on Kauai in the late '90s ultimately led to the new theory: A little more than 6 feet below the surface of the collapsed limestone cave was a sediment layer flavored by the sea: It contained bits of coral, mollusk shells, and sand. Its mouth, however, was about 325 feet from shore and blocked by a 23-foot wall, according to a press release. The paleoecologist who found it couldn't reconcile those figures with Hawaii’s current tsunami evacuation maps, which were based on the 1946 tsunami. That one was triggered by an 8.6-magnitude quake in the Aleutian Islands and would have sent water only about 8 feet up the sinkhole's wall.
But Japan's devastating 2011 earthquake led scientists to do some new modeling. Per the study, they determined that an Aleutian quake with a "moment magnitude of ~9.25" could create a wall of water between 26 and 30 feet—enough to make its way into the sinkhole. Radiocarbon dating of marine deposits from the sinkhole, Sedanka Island in the Aleutians, and along the West Coast indicate a tsunami occurred between 350 and 575 years ago, though the dating doesn't prove it was a single tsunami. Still, geophysicist Gerard Fryer says he's "absolutely convinced it's a tsunami, and it had to be a monster tsunami"; he has worked with Honolulu to update its evacuation maps to account for such a monster. "The revisions to maps are extensive, in some cases twice as far inland," the study's lead author tells Scientific American. (An amazing tsunami story: Family finds girl lost decade ago in a tsunami.)