Day (2001) and Ward and Day (2005) hypothesize that, during an eruption at some future date, the western half of the Cumbre Vieja—approximately 500 km3 (5 x 1011 m3) with an estimated mass of 1.5 trillion metric tons—will catastrophically fail in a massive gravitational landslide and enter the Atlantic Ocean, generating a so-called mega-tsunami. The debris will continue to travel along the ocean floor as a debris flow. Computer modelling indicates that the resulting initial wave may attain a local amplitude (height) in excess of and an initial that approximates to , and travel at about (approximately the speed of a jet aircraft), inundating the African coast in about one hour, the southern coastlines of the British Isles in about 3.5 hours, and the eastern seaboard of North America in about six hours, by which time the initial wave will have subsided into a succession of smaller ones each about to high. These may surge to several hundred metres in height and be several kilometres apart while retaining their original speed. The models of Day et al.; (1999), Ward and Day (2001), suggest that the event could inundate up to inland. If the model is correct, then this scale of inundation would greatly damage or destroy cities along the entire North American eastern seaboard, including Boston, New York City, Miami and many other cities near the Atlantic coast. There is controversy about the threat presented by the Cumbre Vieja. Current indications are that recent landslides may have been gradual and therefore may not generate tsunamis unless they increase in magnitude. Studies of possible local 'mega-tsunamis' in the Hawaiian Islands draw distinctions between the tsunami wave periods caused by landslides and subduction-zone earthquakes, arguing that a similar collapse in Hawaii would not endanger Asian or North American coastlines. Sonar surveys around many volcanic ocean islands including the Canary Islands, Hawaii, Réunion etc., have mapped debris flows on the sea floor. Many of these debris flows, about long and up to thick, contain mega-blocks mixed up with finer detritus. The debris flows are now considered to be a normal process where a volcano sheds some proportion of excess material and thereby makes itself more stable. It is also known to occur on all volcanoes based on the land, in or under the sea. Moore (1964) was the first geologist to interpret such features depicted on a United States Navy bathymetric chart. The