Undersea Noises Become Focus of Titan Submersible Rescue Effort

Unexplained noises detected beneath the waves of the North Atlantic became the focus on Wednesday of an urgent search for five people inside a submersible that disappeared three days earlier during a dive to the wreckage of the R.M.S. Titanic.

The possibility that the sounds might lead to the lost craft offered a slender line of hope to an international team of rescuers who have been scouring a patch of the North Atlantic twice the size of Connecticut and two and a half miles deep. A growing international force of planes, ships and underwater robots was racing to find the 22-foot-long vessel, known as the Titan, and bring it to the surface before its pilot and four passengers run out of oxygen.

When the submersible, owned by OceanGate Expeditions, lost contact with a chartered ship at the dive site on Sunday morning, it was believed to be equipped with only four days’ worth of oxygen. Coast Guard officials said they have been operating under the assumption that they must reach the vessel before Thursday morning for any chance of the crew’s survival.

Capt. Jamie Frederick of the U.S. Coast Guard said in a news conference that two remotely operated undersea vehicles were looking for the source of the sounds, and a team of experts was poring over recordings of the noises to determine whether they were a signal from the missing vessel. On Wednesday afternoon, however, that analysis remained “inconclusive.”

“This is a search-and-rescue mission, 100 percent,” Captain Frederick said. “When you’re in the middle of a search-and-rescue case, you always have hope.”

Canadian surveillance aircraft first detected the underwater noises on Tuesday, using special listening buoys dropped in the search area, and the sounds prompted the authorities to relocate the two remotely operated vehicles to investigate, Captain Frederick said. They had no success.

The noises were detected again by more Canadian planes flying over the area on Wednesday, he said.

Carl Hartsfield, an underwater vehicle designer at the Woods Hole Oceanographic Institution in Massachusetts who is part of the rescue team, said the sounds had been described as “banging noises.” But he cautioned that some natural sounds from undersea animals can sound human-made, and that acoustic experts were still analyzing the recordings.

Captain Frederick added: “We don’t know what they are, to be frank with you.”

Still, percussive sounds from under the sea raised the possibility that someone inside the vessel was making an “improvised signal for locating the vehicle,” said Jeff Eggers, a retired Navy commander with experience piloting compact submersibles.

“There’s lots of things in the ocean that will make noise and be heard on a sonobuoy, but there are few things that will sound like regular banging on metal,” he said.

Submarine crews who are unable to communicate with boats on the surface are taught to pound on the ship’s hull so they can be detected by sonar. Paul-Henri Nargeolet, a former diver and submarine pilot in the French Navy, was among the five missing passengers.

The Titan submersible — a cramped, cylindrical craft with a single porthole, no seats, a tiny, curtained toilet and electric propellers — faced pitch blackness, icy cold and hull-shattering pressure on its descent to see the wreckage of the famous ocean liner that sank in 1912.

Passengers pay up to $250,000 for the experience, which is part of a booming high-risk travel industry. The others on board Sunday included the British billionaire and adventurer Hamish Harding, 58, and a British-Pakistani businessman, Shahzada Dawood, 48, and his son, Suleman, 19, both science buffs.

Stockton Rush, the chief executive of OceanGate and a booster of deep-sea tourism, was piloting the submersible, according to the company. Leaders in the submersible craft industry had warned of possible “catastrophic” problems with the Titan’s development.

The conditions inside the submersible — assuming its hull was not crushed — were probably becoming more dire on Wednesday as oxygen levels dropped, medical experts said.

Captain Frederick said he did not want to speculate on when the search-and-rescue operation might become a recovery effort.

“Sometimes you’re in a position where you have to make a tough decision,” he said. “We’re not there yet.”