The NOAA took 20 years to develop a reliable tsunamograph, an apparatus that provides accurate, real-time data on tsunamis. It consists of an anchored, ocean-bottom pressure recorder and a companion buoy (called DART, for Deep-Ocean Assessment and Reporting of Tsunamis). The recorder, sitting at a depth of up to 5,000 meters, measures changes in pressure due to changes in water level. The recorder transmits acoustic signals to the buoy, which, in turn, relays the measurements of wave height to satellites. This information is then used to forecast the progress of a tsunami. Although each tsunamograph costs a mere $200,000, there are only about 50 in operation worldwide. There are scarcely any in the middle of the Pacific, and practically none in the Indian Ocean. Each dot on the map above represents a single buoy, an object about five feet wide that resembles a flying saucer. Many countries, such as India and Indonesia, have resisted acquiring DARTs from the NOAA out of a sense of "security nationalism." Japan relies on land-based cables that connect to ocean-floor recorders. This is less reliable than buoys, as cables tend to snap when the earthquake fault deforms the seafloor they lie on.
Source: National Oceanic and Atmospheric Administration's National Data Buoy Center