Because tsunamis often start far out in the ocean, it can take hours for the huge wave to reach land. Engineers design specialbuoys (floating devices) that can be left far out to sea to detect tsunami waves early, allowing time for coastal areas to be evacuated. As a tsunami moves across deep ocean water, its wave is usually only a few centimeters high. Sensors placed on the ocean bottom can detect even small tsunamis. The sensor information is sent to a buoy on the ocean surface and then relayed to tsunami centers via satellite. This system is known as DART (deep-ocean assessment and reporting of tsunamis; see photo above). Tsunami buoys are typically placed far away from the coast so that when they detect a tsunami the maximum amount of time is made available to warn coastal residents.