Banging sounds picked up by echo location during a search operation for a stricken nuclear submarine in the Sixties were feared to have been made by surviving crew members - as they knocked on the hull to alert rescuers before all hands were determined lost at sea, newly declassified documents suggest.
Some reports have suggested that some of the 129 crew members on board the American USS Thresher that sank following a major failure at sea in 1963 may have survived for 24 hours before the submarine imploded while rescuers hunted for the vessel.
However, the sounds had been discounted following a US Navy investigation and the US Court Of Inquiry has ruled that none of the suspected signals picked up by one of the vessels in the search, the US nuclear submarine USS Seawolf, equated with any sounds that ‘could have been originated by human beings’.
It ruled that the Thresher had been destroyed almost instantly after sinking as the submarine imploded.
Now, accounts of those involved in the search and rescue attempt have been revealed in 600 pages of newly declassified documents that appear to suggest that the sounds picked up by rescuers made them initially suspect that they could hear feint and garbled signals and tapping noises.
That included what sounded like banging on metal, which the rescue crews believed to have been coming from the crew of the Thresher – a day after it sank.
The USS Thresher had been carrying out test dives more than 200 miles off the American coast around Cape Cod, Massachusetts, on April 10, 1963, with plans to team up with the submarine rescue ship Skylark to start dive trials following a recent overhaul of the submarine.
During the trials, however, as the Thresher submerged to the planned test depth, the Skylark picked up an unclear and garbled communication from the Thresher crew reporting “minor difficulties”.
Despite attempts to contact the Thresher crew again, the communications went silent – sparking a search and rescue operation as personnel on the surface rightly feared the submarine had suffered a major incident and sunk.
Some of those sounds recorded in the subsequent search were picked up on April 11, the declassified documents reveal - some 24 hours after the Thresher had failed to surface - raising fears at the time that some crew members had survived the initial catastrophe and had been making attempts to alert rescue teams.
A range of sounds, picked by echo location and fathometer equipment, had been noted by the USS Seawolf during four dives in the area last recorded for the Thresher’s location before communications were lost.
In one of the accounts revealed in the documents, the Seawolf’s commander reported that his crew could hear what was thought to be “interrupted keying” that may have been dashes, perhaps an attempt at morse-code.
He also reported what were thought to have been distress signal tones including “one very sharp, clear tone, and one fuzzy, modulated signal weaker than the other.”
Following those sounds, the rescue team sent a communication to the Thresher, asking: “Send us a message. Send us a message.”
A series of 37 'pings' had been picked up by sonar, with one record made in the documents suggesting rescuers heard what they thought might be a “very weak voice, unreadable.”
Another communication was then sent out, asking the Thresher to bang five times on the hull and the documents reveal that the Seawolf crew may have heard “loud, spaced” bangs, sounding like metal-on-metal, about eight seconds apart – but no further sounds were recorded following that.
The Seawolf was just one of the US Navy vessels involved in the search and rescue including surface ships from the Naval Research Laboratory which had seep-search acoustic research sonar, plus other Navy vessels such as the submarine USS Sea Owl.
Close sonar searches of the area and searches with a deep-camera system failed to locate the Thresher before she was later determined lost, with all hands lost with her.
Documents recording an investigation by the US Navy into the catastrophe had been classified since the Sixties under what had been determined operational reasons but they have been released following a court case after James Bryant, a retired naval commander of Thresher-class submarines, successfully sued for their release in 2019.
Later searches found the wreckage of the USS Thresher but the submarine had crumpled and split up into six parts, some 8,400ft below the surface on the seabed and none of the crew’s bodies could be recovered.