Last week I took a tour of the Nevada Test Site and while I’ll get to a full description of that shortly, this thread in sci.space.tech was too good to not mention.
A 1957 atomic test called Pascal-B set off a small nuclear explosion in a 500 foot shaft that was covered by a metal lid. A quick thumbnail estimate of the shockwave effect on the metal lid gave a resulting velocity of six times the earth’s escape velocity and thus the legend of the “manhole cover in space” is born. One of the project scientists comments on the whole thing.