[MrNeutron1962]

What kind of yield are you predicting, 40 Km is in the multi-megaton range. You are talking 15 to 20 megatons, this is a very large weapon, who are you proposing is going to have access to such a large device, that poses an immediate threat. Clearly the Russians and Chinese have build, or could build, such a device. The largest deployed weapon the US had, as I recall anyway, was the W53 device for the Titan II, weighing in at 6300 lbs, it had a published yield of 9 megatons.

[backhoe]

Well, my understanding of the tactical warheads mentioned is that they are in the kiloton or sub-kiloton range. Great for cracking Hoover dam, or toppling a few skyscrapers, but certainly not a city buster. ( Make a hell of a mess, however... )

The biggest weapon I can recall offhand was the Soviet test shot of a 58-68 megaton back in the 1960's-- I don't have the info handy, but I believe I put some links to it in some of the other stuff I've posted here... like that India-Pakistan post.