When volcanic mountains slide into the sea, they trigger tsunamis. How big are these waves, and how far away can they do damage? Ritter Island provides some answers. Early one March morning in 1888, a 4-cubic-kilometer chunk of the Ritter Island volcano collapsed into the Bismarck Sea northeast of New Guinea. This volume of land was about twice that of the Mount St. Helens landslide in 1980, and it is the largest historically recorded tsunami-causing volcanic sector collapse. The ensuing landslide triggered a tsunami tens of meters high. The waves were still 8 meters high when they reached parts of...