The tsunami, like all others, behaved differently in deep water than in shallow water. In deep ocean water, tsunami waves form only a low, broad hump, barely noticeable and harmless, which generally travels at high speed of ; in shallow water near coastlines, a tsunami slows down to only tens of kilometres per hour but, in doing so, forms large destructive waves. Scientists investigating the damage in Aceh found evidence that the wave reached a height of when coming ashore along large stretches of the coastline, rising to in some areas when travelling inland. Radar satellites recorded the heights of tsunami waves in deep water: maximum height was at two hours after the earthquake, the first such observations ever made.