Data for megatsunamis are often less than credible, so skepticism about the phenomenon abounds.“In some of the newspapers you read, it looks like I'm forecasting the end of the world is just around the corner,” Ramalho said.

Gisela Winckler, a professor at Lamont and co-author, said, “But there was no way that in this subtropical island in the middle of the Atlantic that there have ever been glaciers.”To confirm their suspicions, Ramalho and his colleagues used radioactive dating to test how long the boulders have been exposed to the sun.

When rocks with olivine are bombarded by cosmic rays, they produce a specific isotope of helium (However, this French study did not predict a megatsunami, but rather waves that were only about 45 feet tall.

This discrepancy is a result of the French study simply looking for evidence in a different place.

Before Ramalho, no one had examined the large, inland boulders.

Previous measurements had been based on debris deposits near the sea level, which were hard to infer wave size from.

There is precedent for Ramalho's theory—other studies have also suggested other prehistoric volcanic collapses that resulted in other megatsunamis—but it is shaky.

“There are several studies since the 1980s putting forth evidence for the impact of megatsunamis, mostly in the The rarity of a megatsunami event limits the amount of data available to use, and previously found tsunami deposits can be destroyed by landscape development in the area.

Lamont-Doherty Earth Observatory investigated the scene of the crime—Fogo volcano, off the coast of Cape Verde.

Their article, published in the journal Science Advances, lays out the case that debris from the volcano caused a megatsunami over 800 feet tall.