[arethuza]

A 3.8 megaton H-bomb exploding at ground level would have created a lot of fallout - most tests were careful to explode high yield weapons high enough that fall out wouldn't be a problem.

Where tests of large H-bombs were done at ground level there were huge amounts of fallout created. However, these tests were usually in very remote places with nobody around in the few hundred miles downwind. However, there were still serious problems with fallout:

http://en.wikipedia.org/wiki/Castle_Bravo