No. 38: Mar-Apr 1985
April 9, 1984. Western Pacific. The crews of three airliners en route from Tokyo to Anchorage observed a gigantic mushroom cloud about 180 miles east of Japan. The cloud was moving rapidly up and away from a cloud layer at 14,000 feet. It eventually reached a maximum altitude of about 60,000 feet, at which time its maximum diameter was about 200 miles. No fireball or flash was seen by anyone.
A nuclear explosion, possibly on a submarine was suspected. One pilot issued a Mayday alert and ordered his crew to don oxygen masks. However, when an F-4 Phantom dispatched from Japan arrived at the scene, it detected no abnormal levels of radioactivity. Wake Island hydrophones, to the southeast, detected some submarine volcanic activity far south of the cloud, but no detonations in the area the cloud was spotted. The distance of the volcanic disturbances and the prevailing winds ruled out volcanic sources of smoke. In the absence of any hydrophonic evidence, the authors concluded that the mysterious cloud came either from a man-made atmospheric explosion (a huge one!) or some as yet unknown natural phenomenon.
(Walker, Daniel A., et al; "Kaitoku Seamount and the Mystery Cloud of 9 April 1984," Science, 227:607, 1985.)