I say it's cumulus cloud forming over intense fire from which heated air is rising -- common phenomenon -- not someone testing 'suitcase' atom bombs after all.
Note - Yes, we've seen high-rising smoke from fires do this on occasion. However, if the witnesses are accurate, that would not explain the three 'explosions' heard before the cloud formed. No one is saying anything about 'suitcase atom bombs' being the cause...which remains unknown. If there was a large wildfire in the area, the locals would know about it. -ed
From Gary Toll
Thought you might be interested in this picture taken from the area of Pahrump, Nevada, looking back toward Las Vegas, Nevada. A classic mushroom cloud formed very rapidly, from a brush fire in between the mountains of Vegas and Pahrump. I keep my eyes open all the time toward that area, and I hope that I do not see one of these rise over the ashes of Las Vegas due to one of the suitcase nuclear weapons the homeland security keeps talking about and also has been in the news.