“Underwater images of the seabed surrounding the Hawaiian Islands show that they are surrounded by huge aprons of debris shed from their volcanoes over tens of millions of years,” the writer Bill McGuire wrote in his book, A Guide to the End of the World. “Within this great jumbled mass of volcanic cast-offs, nearly 70 individual giant landslides have been identified.”

In at least one such landslide, a 1,000-foot mega-tsunami slammed into the island of Lanai. A wave that big on Oahu today would almost certainly wipe out Honolulu. But scientists can’t say for sure how—or, critically, when—such a catastrophe would play out. That’s largely because no one in recorded history has seen one of these things. “The lack of direct observations means that little is still known on the mechanics of collapse development,” Ramalho and his colleagues wrote in their paper.

Most tsunamis are generated from tectonic activity. For instance, huge earthquakes triggered the two most destructive tsunamis in recent history: the 2011 Japan tsunami and the 2004 Indian Ocean tsunami. In both cases, record wave height topped out between around 100 feet and 130 feet.

Now imagine a tsunami five or even 10 times that size.

Most scientists agree that a catastrophic flank collapse will generate an unimaginably massive tsunami again someday, but they’re cautious about guessing when it might happen. A popular ballpark estimate: maybe sometime within the next 100,000 years. Whatever the case, a volcanic-flank collapse in Hawaii would generate a series of giant tsunamis that would likely destroy cities in several countries, including in the United States, Canada, Japan, and China, McGuire says. “In deep water, tsunamis travel with velocities comparable to a jumbo jet,” he wrote, “so barely 12 hours will elapse before the towering waves crash with the force of countless atomic bombs onto the coastlines of North America and eastern Asia.”

The hazard of these “very low-frequency, very high-impact” geologic events should not be underestimated, Ramalho told me, but the potential for such catastrophes shouldn’t cause panic either. “We better improve our resilience to their impacts,” he said. “We should improve our monitoring capabilities of possible volcanic sources, we should do more research on the subject, and we should—rationally and cooly—think of what can be done to mitigate the possible impacts of such an event.”

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