The massive earthquake off the coast of Japan in March caused a rare "merging tsunami" in which two waves combined to amplify the destruction after landfall, according to NASA. For the first time ever, US and European radar satellites captured images of the two wave fronts, confirming the existence of the long-hypothesized process, which forms a "single, double-high wave far out at sea." "This wave was capable of traveling long distances without losing power. Ocean ridges and undersea mountain chains pushed the waves together along certain directions from the tsunami's origin," NASA said in a statement on its website. "The discovery helps explain how tsunamis can cross ocean basins to cause massive destruction at some locations while leaving others unscathed," it said, adding that the research could help to improve forecasts. "It was a one in 10 million chance that we were able to observe this double wave with satellites," said Y. Tony Song, of NASA's Jet Propulsion Laboratory, which partnered with Ohio State University for the study. "Researchers have suspected for decades that such 'merging tsunamis' might have been responsible for the 1960 Chilean tsunami that killed about 200 people in Japan and Hawaii, but nobody had definitively observed a merging tsunami until now." The 9.0-magnitude underwater earthquake and tsunami on March 11 left 20,000 people dead or missing, devastated large areas of northeastern Japan and sparked a nuclear crisis at the Fukushima nuclear plant.