The manhole cover legend comes from 1957, Operation Plumbbob during the Pascal B test shot. This was an underground nuclear weapons test wherein the borehole had a steel lid (the manhole cover) welded to the well casing.
Look up the Pascal B shot here for a short description of the manhole cover story: http://nuclearweaponarchive.org/Usa/Tests/Plumbob.html
Dr Brownlee offers his own account of the event here: http://nuclearweaponarchive.org/Usa/Tests/Brownlee.html
Quoting Dr Brownlee in part:
For Pascal B, my calculations were designed to calculate the time and specifics of the shock wave as it reached the cap. I used yields both expected and exaggerated in my calculations, but significant ones. When I described my results to Bill Ogle, the conversation went something like this.
Ogle: "What time does the shock arrive at the top of the pipe?"
RRB: "Thirty one milliseconds."
Ogle: "And what happens?"
RRB: "The shock reflects back down the hole, but the pressures and temperatures are such that the welded cap is bound to come off the hole."
Ogle: "How fast does it go?"
RRB: "My calculations are irrelevant on this point. They are only valid in speaking of the shock reflection."
Ogle: "How fast did it go?"
RRB: "Those numbers are meaningless. I have only a vacuum above the cap. No air, no gravity, no real material strengths in the iron cap. Effectively the cap is just loose, traveling through meaningless space."
Ogle: And how fast is it going?"
This last question was more of a shout. Bill liked to have a direct answer to each one of his questions.
RRB: "Six times the escape velocity from the earth."
Bill was quite delighted with the answer, for he had never before heard a velocity given in terms of the escape velocity from the earth! There was much laughter, and the legend was now born, for Bill loved to report to anybody who cared to listen about Brownlee's units of velocity. He says the cap would escape the earth. (But of course we did not believe that would ever happen.)
The next obvious decision was made. We'll put a high-speed movie camera looking at the cap, and see if we can measure the departure velocity.
In the event, the cap appeared above the hole in one frame only, so there was no direct velocity measurement. A lower limit could be calculated by considering the time between frames (and I don't remember what that was), but my summary of the situation was that when last seen, it was "going like a bat!!"
So there you have it, the genesis of the Project Thunderwell legend.