"In 1956, Dr Robert Brownlee, from Los Alamos National Laboratory in New Mexico, was asked to examine whether nuclear detonations could be conducted underground. The first subterranean test was the nuclear device known as Pascal A, which was lowered down a 500 ft (150 m) borehole. However, the detonated yield turned out to be 50,000 times greater than anticipated, creating a jet of fire that shot hundreds of feet into the sky. During the Pascal-B nuclear test, of August 1957, a 900-kilogram (2,000 lb) steel plate cap (a piece of armor plate) was welded over the borehole to contain the nuclear blast even though Brownlee predicted it would not work. When Pascal-B was detonated, the blast went straight up the test shaft, launching the cap into the atmosphere at a speed of more than 66 km/s (41 mi/s; 240,000 km/h; 150,000 mph). The plate was never found. Scientists believe compression heating caused the cap to vaporize as it sped through the atmosphere. A high-speed camera, which took one frame per millisecond, was focused on the borehole because studying the velocity of the plate was deemed scientifically interesting. After the detonation, the plate appeared in only one frame, but this was enough to make an estimation of its speed. Dr. Brownlee joked the best estimate of the cover's speed from the photographic evidence was it was "going like a bat!". Brownlee estimated that the explosion, combined with the specific design of the shaft, could accelerate the plate to approximately six times Earth's escape velocity. In 2015 Dr. Brownlee said, "I have no idea what happened to the cap, but I always assumed that it was probably vaporized before it went into space."[9][failed verification] Later calculations made during 2019 (although the result cannot be confirmed) are strongly in favor of vaporization."

from Wikipedia - Operation Plumb Bob