The sound that was heard during the search by rescuers in the area where the Titan submersible was located has been revealed.

The submersible disappeared during an expedition to the Titanic wreck on Sunday 18 June 2023.

The “hollow” sound, played for the first time in a documentary, has a regular, steady rhythm that caught the attention of experts – and had raised hopes it could be an SOS signal emitted by the five occupants on board.

The sound can be heard at 0:20 of the promo video clip below.

“It could be someone knocking, the symmetry between these knocks is very unusual,” former Navy submarine captain Ryan Ramsey points out in the documentary, among others.

“It’s rhythmic, it’s like someone is making this sound and the fact that it repeats is really unusual,” adds the former captain

The new documentary, to be released to mark the one-year anniversary of the tragedy in June, looks behind the scenes of the multi-million dollar search operation.

The documentary, from ITN productions for Channel 5, also asks what lessons can be learned from the disaster.

The Titan is an experimental vessel and the participants sign a document which mentions the possibility of death three times on the first page.

Among the passengers were Pakistani billionaire Shahzada Dawood, 48, and his 19-year-old son, Suleiman.