On April 9, 1984, the captain of a commercial airplane en route to Anchorage from Tokyo was so alarmed by a huge mushroom-like cloud looming off the coast of Japan that he issued a Mayday alert and put his crew on oxygen as a precaution. The crews of two other flights that night also saw the strange cloud that eventually spanned 320 kilometers in diameter.
According to Daniel Walker and colleagues at the University of Hawaii at Manoa, Honolulu, one possible explanation might be the explosion of a nuclear submarine. But no fireball was seen and neither the planes nor the dust from the cloud showed unusual levels of radioactivity. To look for evidence of explosions of any kind, Walker’s group analyzed data from an array of ocean-bottom hydrophones located southeast of the cloud site. They found no explosion evidence but they did discover that some undersea volcano–probably the Kaitoku Seamount located to the southwest of the cloud–had displayed a peak in activity on April 8 and 9. While no plume had been observed in the Kaitoku area during that time, the researchers reasoned that one might have been released at night, then carried northward 1,470 km across the Pacific. When they studied the wind data for those days, however, they found that such a plume would have to have traveled to the southeast instead. Moreover, they concluded that no natural mechanism could have caused the cloud to rise as fast as it did, other than a volcanic eruption directly below the cloud site–an idea ruled out by the hydrophone data. And so the mystery continues.
Walker’s group writes in the Feb. 8 SCIENCE: “…our analyses indicate that the mystery cloud was produced either by an as-yet-unknown natural phenomenon or by a man-made atmospheric explosion.” If the cloud was natural, says Walker, it’s important for people with their finger on the red button to know that bombs are not the only cause of mushroom-shaped clouds.