Mushroom cloud from the detonation of Mike, the first hydrogen bomb to be successfully tested, during Operation Ivy. Image date: October 31, 1952. The thermonuclear device was fired in the Pacific Proving Grounds, over Elugelab Island in the Enewetak atoll of the Marshall Islands. The nuclear fusion-based device obliterated the island. Two minutes after Zero Hour, the cloud rose to 40,000 feet. Ten minutes later, as it neared its maximum, the cloud stem had pushed upward about 25 miles, deep into the stratosphere. The mushroom portion went up to 10 miles, and spread for 100 miles.