Listeners can now hear the mysterious banging noises that echoed through the Atlantic Ocean shortly after contact with OceanGate’s Titan submersible went dark.

The sounds, which gave a flicker of hope that those inside the craft might be saved, can now be heard in an audio clip featured in a documentary titled “The Titan Sub Disaster: Minute by Minute,” the New York Post reported Wednesday.

While crews were searching for the underwater vessel two days after they lost contact during its journey to the wreck of the Titanic, the searchers heard a knocking sound.

“Late in day two, CNN reports that the Canadians heard banging noises at 30-minute intervals from under the ocean, and that changed everything,” one man speaking during the documentary explained.

The video clip then plays the banging sound:

“But it was later determined that the noise couldn’t have been the passengers and crew as they died instantly in the catastrophic implosion the sub suffered two hours into its voyage,” the Post article read, noting the source of the noise remains unknown.

There was speculation the noise was made by one of the Titan’s passengers who was trying to send rescuers a message. However, authorities said the sound may have come from the ocean itself or nearby ships.

The lead submersible in the expedition later found the debris of the Titan on the ocean floor laying approximately 1,600 feet from the Titanic.

In October, more “presumed human remains” were recovered from the site where the Titan’s debris lay, according to the U.S. Coast Guard (USCG). Breitbart News reported at the time:

The submersible, which imploded shortly after embarking on an expensive mission to view the sunken Titanic in June, was manned by 61-year-old Stockton Rush, CEO of the diving company OceanGate. He was killed instantly, as were the four passengers on board: French submariner Paul-Henri Nargeolet, 77; British billionaire and explorer Hamish Harding, 58; Pakistani businessman Shahzada Dawood, 48; and his 19-year-old son, Suleman Dawood.

In July, OceanGate announced it was suspending all of its “exploration and commercial operations” after the Titan imploded.