[95Tarheel]

I could be wrong, but I think the reason is that the biggest explosion the US ever set off was more like 15 Mtons. (Castle... Bravo, I think) Therefore, a US picture of a 51 Mton explosion doesn't exist. (I think the US might have made some bombs that big though.) The Soviets detonated the largest device, and I think the estimates are it was 50+ Mtons. If anyone knows of where I can get a picture of this, I would be grateful!

As an aside, I've read that there is no practicality to building a weapon capable of anything more than 200 Mtons b/c the additional explosive force cannot practically be directed toward the earth's surface, and would instead blow a hole in the atmosphere.

Cool pictures- I'm saving some!