We've only ever lost a few subs with nuclear weapons onboard.

Robert Ballard became famous as the explorer who found the wreckage of the Titanic. But what most people don't know about that expedition is that Ballard also secretly worked with the U.S. Navy to learn more about two lost nuclear submarines.

Ballard shared that nugget when he came to the Tampa Bay area last week for the Coastal Cities Summit, organized by the University of South Florida and other institutions. He spoke with the St. Petersburg Times about the Titanic and his underwater exploration career.

I can only tell you now because they declassified this a few months ago. The Navy was not interested in the Titanic. … I mean, they funded the technology because it had so many military applications. And I was a naval intelligence officer for 30 years, and so I did a lot of missions for the Navy. Many remain classified, my best stuff. Rats …

Yes, the Titanic was a cover for a series of military operations. The Titanic was here, and over here was the Scorpion and over here was the Thresher (as he says this, he arranges three objects on a tabletop, roughly in a line, the center one depicting the Titanic).

And had that not occurred, I probably would not have found the Titanic because they wouldn't have funded me. I mean, if the Titanic was in the Indian Ocean, it'd probably still be in the Indian Ocean. But … it was straddled by two very interesting subs that we had lost — and the Scorpion was lost on war patrol … and it was carrying nuclear weapons. So it was a very hot sub to the Navy …

So the Navy said, look, we never actually tracked that all down, we never found the reactors, we never did environmental studies on them, nor did we ever find the weapons. We'll fund you to build all this stuff and we'll fund you to find the Titanic, although we don't expect you to find it because we're going to have you work most of the time on these subs and give you very little time to actually find the Titanic. I said, I'll take whatever I can get. - tampabay

Some info on the USS Scorpion:

Scorpion carried two nuclear-tipped Mark 45 anti-submarine torpedoes (ASTOR) when she was lost. The warheads of these torpedoes are part of the environmental concern. The most likely scenario is that the plutonium and uranium cores of these weapons corroded to a heavy, insoluble material soon after the sinking, and they remain at or close to their original location inside the torpedo room of the boat. If the corroded materials were released outside the submarine, their large specific gravity and insolubility would cause them to settle down into the sediment.