According to urban legend, a manhole cover was accidentally launched from its shaft during an underground nuclear test in the 1950s, at great enough speed to achieve escape velocity. The myth is based on a real incident during the Operation Plumbbob nuclear tests, where a 900 kg steel plate cap was blasted off the test shaft at an unknown velocity, and appears as a blur on a single frame of film of the test; it was never recovered. A calculation before the event gave a predicted speed of six times Earth escape velocity, but the calculation is not likely to have been accurate. 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!!"[11][12]