Operation Plumbbob was a series of nuclear tests conducted between May 28 and October 7, 1957, at the Nevada Test Site, following Project 57, and preceding Project 58. It was the biggest, longest, and most controversial test series in the continental United States.
During the Pascal-B nuclear test, a heavy (900 kg) (approx 1,984 pounds) steel plate cap (a piece of armor plate) was blasted off the top of a test shaft at a speed of more than 66 km/s (41 miles per second or 147,600mph). Before the test, experimental designer Dr. Brownlee had performed a highly approximate calculation that suggested that the nuclear explosion, combined with the specific design of the shaft, would accelerate the plate to six times escape velocity. The plate was never found, but Dr. Brownlee believes that the plate never left the atmosphere (it may even have been vaporized by compression heating of the atmosphere due to its high speed). The calculated velocity was sufficiently interesting that the crew trained a high-speed camera on the plate, which unfortunately only appeared in one frame, but this nevertheless gave a very high lower bound for the speed. After the event, Dr. Robert R. Brownlee described the best estimate of the cover's speed from the photographic evidence as "going like a bat out of hell!" The use of a subterranean shaft and nuclear device to propel an object to escape velocity has since been termed a "thunder well".
This incident was reputedly used as part of the technical justification for the Orion project for possible use of nuclear blasts for outer-space propulsion.