can you contextualise this? I thought it was about the flying manhole cover

During the Pascal-B nuclear test of August 1957,[8][9] a 900-kilogram (2,000 lb) steel lid was welded over the borehole to contain the nuclear blast, despite Brownlee predicting that it would not work.[8] When Pascal-B was detonated, the blast went straight up the test shaft, launching the cap into the atmosphere. The plate was never found. In a conversation with Bill Ogle, Brownlee estimated its velocity as "six times the escape velocity from the Earth"—approximately 67.2 km/s (150,000 mph).[10]

https://en.wikipedia.org/wiki/Operation_Plumbbob#cite_ref-Brownlee,_Harrington_9-0

hence this thing being posted here more than once.