Lurker mmmmbacon sends this along and I found it as captivating as (s?)he does. Like most of us I've seen a lot of historical images of mushroom clouds, but I can't remember one where the "stem" looked like that. To imagine being anywhere near it! I'll just paste in mmmmbacon's fine description, too, thanks a lot!
It's a picture of the second of the Operation Crossroads nuclear tests at the Bikini atoll, on July 25, 1946, the first nuclear operations conducted in the Marshall Islands. Two bombs were exploded; one an atmospheric burst (named Able) on July 5th, and the second, an underwater detonation (Baker), which is what the picture is of. The ships in the picture are unmanned target ships, of which Able sank five and Baker sank eight. Looking at the picture, I can't imagine how they weren't all sank. The blast caused a tidal wave to pound the atoll and significant damage was done from that.
The ships that didn't sink were decontaminated by navy personell at a time when radioactivity was poorly understood. This became a controversy as many questioned whether navy personnel's lives were endangered. All the water from the blast was radioactive.
Scuba divers today enjoy the Bikini atoll for all the ships that were sank in Operation Crossroads.
Here's a good background page:
http://www.history.navy.mil/faqs/faq76-1.htm
Sound good oil paintings of Baker.
http://www.history.navy.mil/ac/bikini/bikini5.htm
Here's two great online galleries of lots of nuclear test blasts.
http://www.wps.com/archives/wxvax7.esa.lanl.gov/
http://www.zvis.com/nuclear/nukimgdht.shtml