La Palma Island of the Canary Islands has a weakened land mass on one of it's steep shores. Weakened by a 1949 volcano (the Cumbre Vieja), it nearly experienced a complete collapse. Such a collapse would create a mega tsunami. Taking approximately 5 hours to reach the eastern shores of the United States (travelling at 725 km per hour), it would destroy everything up to 12.5 miles inland from major cities such as New York all the way down to Miami.
A valid threat from the next volcanic eruption, scientists believe it may take up to five incidents before the collapse will occur...or just one. They say, with a straight face, that they just don't know. The giant landslide would slam into the ocean and result in a mega tsunami. The wave of the mega tsunami would tower higher than the tallest sky scraper in the world. These eruptions seem to happen every 100 years or so (the last being in '49) so don't cancel your vacation plans or head to middle America just yet.
Swiss and other world scientists warn that it is not a question of "if" it is a question of "when."
(Data gathered from this evening's special on the Discovery Channel.)