NOT A KNOWN NATURAL PHENOMENON
Further, the story does not end there.
A geophysicist at the University of Hawaii, Dr. Daniel A. Walker, together with his colleagues monitor an array of hydrophones on the ocean bottom, recording signals such as seismic incidents, volcano eruptions, etc.
Dr. Walker and his colleagues performed a comprehensive analysis of the data recorded by the hydrophones in and around the time of the incident in question. The data shows the absence of any natural seismic or volcanic event that could have caused the April 9th phenomenon off the coast of Japan.
He and his colleagues concluded that it was either an as yet unknown natural phenomenon, or a man-made phenomenon.
report is published in Science. (Walker et al, "Kaitoku Seamount and
the Mystery Cloud of 9 April 1984," Science, 227(4687), Feb. 8,
1985, p. 607-611.)