Geologists have concluded that 73,000 years ago, a tsunami six times taller than the wave that hit Japan in 2011 struck off the coast of West Africa, in the Cape Verde islands.

The tsunami was so powerful that it hurled elephant- and truck-sized boulders more than 600 feet uphill and onto a island plateau.

Scientists had discovered these basalt and limestone rocks a few years ago—but, as is often the case in geology, they weren’t sure how they could’ve have ended up so far from their original formation place. All they knew was that 73,000 years ago, a massive chunk of an ancient volcano on the island of Fogo had collapsed and fallen into the sea. On the nearby Santiago Island, evidence of debris from an ensuing tsunami suggests that the event had wide-reaching geologic effects. But until now, researchers didn’t know how big the tsunami was, nor how many of them occurred.

The giant boulders, though, provide a missing link. The team, comprised of researchers from Columbia University’s Lamont-Doherty Earth Observatory, concluded that the 49 boulders (weighing up to 700 tons) they studied could only have been lifted to their current position by a colossal tsunami. The researchers report their geologic interpretation in the journal Science Advances .