A huge tsunami apparently struck an island off the coast of Africa in prehistoric times—and if so, people living on islands and coastlines today should take care for their safety. According to a new study, a tsunami over 50 stories high swept through the Cape Verdean island of Santiago about 73,000 years ago and tossed around boulders weighing as much as 770 tons, LiveScience reports. The apparent cause isn't too unusual: A volcanic landslide, or so-called "flank collapse" that occurs when a volcano's slopes give way, notes the Guardian. Yet the volcano, called Fogo, is 34 miles away, and scientists have been unsure whether volcanic collapses can cause tsunamis that travel so far across open seas. In fact, lead author Ricardo Ramalho was baffled when he saw the gigantic boulders so far inland on Santiago.
As Popular Science puts it, how could "enormous, truck-sized hunks of striated limestone and submarine basalts" be found up to 2,000 feet inland when the boulders matched rocks on the island's shore? "We got really excited when we realized that the only way to explain the origin of those boulders was with a massive tsunami impact," Ramalho says. His team dated the event based on isotopes in the boulders—which change while the rock is exposed to open air—and said Fogo, which remains active, may be to blame. Now they're analyzing whether volcanic collapses might trigger other long-range tsunamis. "There is a growing awareness that extreme geohazards such as these will eventually, one day happen," Ramalho says, "so we might as well coolly and realistically see what can be done to mitigate their effects." (Visit an island near India, and "you may not leave alive.")