>The Mk-6 was the most terrible weapon ever created by the United States. Inside the Mk-6 was the W-53 thermonuclear warhead which weighed 8850 lb. and had a yield of 9 megatons. Supposably the Soviets had operational warheads even bigger than the W-53, which is pretty hard to believe.

I'm afraid even we deployed some larger weapons. Most notably, the B41 bomb carried by B-52 aircraft, which supposedly had a yield around 20 to 25Mt. We built 500 or so of them, though they'd been retired by the mid-1970s.

The Russians built big. They had a number of 10-20 megaton-class warheads deployed on their missiles.

The largest nuke ever tested was a Russian model that produced a yield of more than 50 megatons (about 3,000 Hiroshima bombs), but, that was a test of a stripped-down model. The design was intended to produce 150 megatons -- enough to make a crater a couple of miles across. It was too big to put on a missile, and too big to carry feasibly on any of their bombers, so the estimates are that they produced only a couple of them. Nicknamed the "Tsar Bomb", in the long Russian tradition of things that are too big to use -- the "Tsar Bell" that cracked before it was ever rang, the "Tsar Cannon" that was too big to fire (or really even move), etc.

For those of you who are thankfully unaccustomed to thinking of terms of megatons (this is actually closer to what I do professionally than Martian geology), Meteor Crater in Arizona was probably 20 to 40 megatons (though I've seen estimates from 5 to 200), Mt. St. Helens' 1980 blast something like 5 megatons.