For years, scientist Bill McGuire of University College London has been warning the world about Cumbre Vieja, a volcano on the island of La Palma off the African coast. If it erupts, he says, it could force a Manhattan-size rock to splash down into the Atlantic Ocean, kicking up 300-story-high waves that would travel out from the island at jet speeds. Within nine hours, 85-foot-high swells would slam Florida's Cape Canaveral. Now scientists at the University of Hawaii say that this wild scenario has historical precedence: A similar event unfolded 120,000 years ago in the Pacific. The researchers found that 120 cubic miles of rock broke off the Mauna Loa volcano, sending an 820-foot tsunami racing across the ocean.