"I've not read up on it lately, but I believe you have to be underground by a considerable distance to be safe these days, on the order of thirty feet or more."
At Ground zero, sure. Five or more miles from a 10 MT airburst, though, just a few feet. It's best to be flat underground, though, you don't want to be "sticking up" into the debris/blast wave.
Thanks... that's pretty much what I thought. I didn't realize the prone part was significant though, if underground.