A massive tsunami approximately seventy-three thousand years ago deposited a scattering of enormous boulders on the Cape Verdean island of Santiago. PHOTOGRAPH BY SADATSUGU TOMIZAWA / AFP / GETTY

Geologists tend to notice when rocks, especially big ones, are out of place. A few years ago, a group of researchers working in the Cape Verde islands, off West Africa, made a particularly jarring discovery—elephant-sized chunks of basalt and limestone that had formed at sea level and somehow ended up more than six hundred feet above, on a volcanic plateau on Santiago Island. The rocks constituted only the final few pages of an unusual narrative. What remained for the scientists, Ricardo Ramalho and his colleagues at Columbia University’s Lamont-Doherty Earth Observatory, was to reconstruct the plot. Since it isn’t easy to move more than seven hundred tons of stone at a time uphill, there were only a few possible protagonists. The one that they settled on was larger than life: a tsunami some eight hundred feet high, almost six times taller than the wave that hit Japan’s Pacific Coast in 2011.

Back in the early nineteenth century, when geology was just emerging as a distinct science, such catastrophic explanations for modern-day features were fairly common. In those years, geologists in Europe and the Great Lakes region of North America began to take note of so-called erratic boulders, which were composed very differently from the local bedrock on which they rested. Monoliths of granite sat, illogically, on limestone; slabs of schist, improbably, on sandstone. The most reasonable interpretation of these foreign rocks, in the context of the contemporary understanding of Earth’s history, was that they had been washed in by the waters of the Great Flood of Noah. Geologists called such flotsam “drift,” and an early version of the geologic time scale included a period known as the Antediluvian—that is, “before the deluge.”

Within a few decades, however, the maturing field of geology had distanced itself from Biblical accounts of Earth’s evolution, and the incongruous rocks were reinterpreted as glacial hitchhikers, remnants of the Ice Age. Gradualistic explanations for geologic phenomena, informed by processes that could be observed today, became de rigueur; invoking the Noachian flood—or any catastrophic origin for rocks or landscapes—became a geologic taboo. (Even so, a vestige of the old flood paradigm persists in the name of an unglaciated region along the Upper Mississippi Valley: the Driftless Area.) The taboo finally lost some of its power in the nineteen-eighties, when it began to seem probable that a meteorite impact at the end of the Cretaceous Period had wiped out the dinosaurs. This unleashed into the scientific literature a pent-up flood of neocatastrophist papers, which sometimes veered toward the sensationalism of tabloids (with references cited, of course). Geologists were at last free to suggest that calamitous but infrequent events—powerful deluges, colossal landslides, gargantuan volcanic eruptions, supersonic impacts from extraterrestrial objects—did in fact play a role in shaping our world. Bad things sometimes happen to good planets.

In the past thirty years, the initial giddiness over extreme geology has subsided and the scientific standard for documenting ancient disasters has risen. When Ramalho’s team began developing their theory about the Cape Verde rocks, there was a heap of skepticism to dispense with. At a latitude of just fifteen degrees north of the equator, the islands were never glaciated. This meant that the only other possible anti-gravity agent was water—namely, a very high and very powerful wall of it. The terrifying force of earthquake-generated tsunamis has already been seen twice in this new millennium, in Indonesia and Japan. But both of those island chains, unlike Cape Verde, lie hard up against the boundaries of tectonic plates, which occasionally lurch into motion, displacing immense volumes of seawater. Cape Verde is comparatively far from the edge of the African Plate. Whence, then, the wave?

One of Santiago’s boulders, composed of basalt and limestone, sitting six hundred feet above sea level. PHOTOGRAPH COURTESY KIM MARTINEAU / LAMONT-DOHERTY EARTH OBSERVATORY PHOTOGRAPH COURTESY KIM MARTINEAU / LAMONT-DOHERTY EARTH OBSERVATORY

Submarine earthquakes are not the only way to generate tsunamis. The sudden addition of a large mass to a body of water—like a chubby bully doing a cannonball into a swimming pool—can generate local waves of extraordinary height. In Cape Verde, the bully must have been an enormous quantity of rock, displaced suddenly from a significant elevation into the waters nearby. And there was a good candidate: the neighboring island of Fogo, thirty miles to the west, which has the shape of a half-eaten chocolate kiss. Whereas its west flank is smoothly conical, its east side is a ragged depression, pulled apart from the rest of the island in a great landslide. But did the slide happen slowly, as on the east flank of Kilauea, in Hawaii, which is collapsing under its own weight, or in a single cataclysmic failure?

Previous geological work on Fogo Island had constrained the timing of the slope failure to between a hundred and twenty-four thousand and sixty-five thousand years ago. Geologists routinely use naturally occurring radioactive isotopes—potassium-40, rubidium-87, uranium-238—to determine the ages of rocks. The question in this case, however, was not the age of the rocks but how long they had been lying in their present anomalous positions. Ramalho and his colleagues used an innovative new method of isotopic dating to establish a link between the landslide on Fogo and the boulders on Santiago. The technique relies on the fact that Earth is bombarded, constantly and invisibly, by cosmic rays—high-energy particles that hurtle toward the planet from deep space, mainly from distant supernovae. When these guts of ancient stars collide with atoms on rock surfaces, they produce small amounts of certain distinctive isotopes. Measure how much helium-3, for instance, has accumulated on a rock face and you know how long it has been exposed to cosmic radiation. In the case of the Santiago boulders, that was about seventy-three thousand years—well within the time window of the Fogo collapse.

So the rock-throwing protagonist has been conclusively identified, but what does the story mean for us? Is it a cautionary tale, a portent of looming disaster that should be added to our list of late-night worries? Probably not. Although the landslide on Fogo and the ensuing megatsunami would have been utterly devastating to nearby islands, it is not clear how far such a locally generated wave would have gone before dissipating. And, more to the point, the obsession with unlikely catastrophes is itself dangerous if it blinds us to the slow-motion disasters that are elapsing before our eyes. Envisioning a single deluge is easy; it takes imagination to perceive a sea change.