Paleotsunami detectives hunt for ancient disasters

Gigantic tsunamis have been decimating coastlines since time immemorial. We ignore these prehistoric warnings at our own peril.

A boulder weighing more than 40 tonnes sits on the sand high above the ocean. Dwarfing every other rock in view, it is conspicuously out of place. The answer to how this massive outlier got here lies not in the vast expanse of the Atacama Desert behind it but in the Pacific Ocean below. Hundreds of years ago, a tsunami slammed into the northern Chilean coast—a wall of water 20 meters high, taller than a six-story building, that swept boulders landward like pebbles.

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As scientists expand their search, they have continued to find ancient tsunamis bigger than those found in historical records, says James Goff, a paleotsunami researcher at the University of Southampton in England. The implications are clear: if a huge tsunami happened once in a given location, it could happen again. The question is whether we’re prepared for it.

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The easiest way to tell that a tsunami hit hundreds or thousands of years ago is to look underground, Goff says. When the wave recedes, it leaves traces of everything it contained strewn across the surface. This thin layer of silt, rocks, tiny shells, and other marine deposits gets buried over time, preserving the tsunami’s path between layers of sediment. In some places, the layers are so well preserved that researchers can see evidence of multiple tsunamis stacked on top of each other like a layer cake.

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