Authorities are unsure about the cause of underwater banging as experts investigate if the sounds originated from the Titanic submarine.

The Mystery of the Missing Titanic Submersible

The U.S. Coast Guard is still trying to determine if the underwater noises picked up by sonar planes Tuesday came from the missing Titanic submersible.

The search-and–rescue teams detected “banging” sounds in 30-minute intervals near where the five passengers aboard the Titanic sub went missing. During a Wednesday afternoon press conference, Coast Guard Captain Jamie Frederick said that a team of experts is listening to recordings of the “banging” sounds picked up by search and rescue teams but added, “We don’t know what they are, to be frank with you.” At the news conference, Carl Hartsfield of the Woods Hole Oceanographic Institution said it’s possible for underwater animals to make noises that sound like they came from humans.

A Canadian airplane picked up the “underwater noises” on Tuesday, resulting in the search team focusing their operation on the area where the sounds came from, according to Frederick. There are five “surface assets” and two remotely operated vehicles searching for the submersible, the Titan. Five more assets are expected to join the search in the next two days, Frederick said, according to The New York Times. Authorities are also using airplanes and sonar buoys to look for the missing sub.

The Coast Guard captain said that the search operation was still “a search-and-rescue mission 100%,” adding that “We need to have hope.”

According to Coast Guard estimates, the five passengers aboard the Titan have less than 24 hours of oxygen remaining. Rescue teams initially were searching an area roughly the size of Connecticut after the vessel went missing, and a last-ditch effort began Wednesday involving heavy machinery and submarines that arrived in Newfoundland on Tuesday night.

Three C-17 aircraft from the U.S. Air Force reportedly landed at a cargo terminal in St. John’s, Newfoundland, carrying unmanned vehicles capable of going 19,000 feet underwater as well as two heavy-duty Hyundai winches emblazoned “6000 kg line pull,” a huge roll of cable, and two large machines that said “high voltage” on their sides, The Daily Mail reported. A forklift truck loaded the equipment onto six flatbed trucks.

The equipment was taken to a port where a ship dubbed the Horizon Arctic was scheduled to leave at midnight, although the journey to the area where Titan is submerged would take 15 hours. The Titan may be as deep as 12,000 feet below the surface and weighs 10,432 kg, so both winches would be necessary to pull it out.

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Those on board the missing Titanic submersible have been identified as billionaire Hamish Harding, British-Pakistani businessman Shahzada Dawood and his son Suleman, Paul-Henri Nargeolet, a French maritime expert, and OceanGate CEO and founder Stockton Rush.

Hank Berrien contributed to this report.