Follow along for live updates on the submersible that vanished while taking five people down to the wreck of the Titanic.

The Coast Guard says it is bringing in more ships and underwater vessels to search for a submersible missing in the North Atlantic after underwater sounds were detected, providing a glimmer of hope three days after the Titan disappeared while taking five people down to the wreck of the Titanic.

Although the exact location and source of the sounds were not yet determined, they allowed searchers to focus on a more narrowly defined area. The full scope of the search was twice the size of Connecticut and 2 1/2 miles (4 kilometers) deep, said Capt. Jamie Frederick of the First Coast Guard District.

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

But even those who expressed some optimism warned that many obstacles remain: from pinpointing the vessel’s location, to reaching it with rescue equipment, to bringing it to the surface - assuming it’s still intact - before the passengers’ oxygen supply runs out.

The U.S. Coast Guard did not elaborate on what rescuers believe the noises could be. The vessel hasn’t been heard from since Sunday. The passengers are estimated to have as little as a day’s worth of oxygen left.

What to know:

• What we know so far about the submersible, what may have gone wrong, and what’s being done to find it

• A look at the five passengers aboard the vessel

• A 2018 lawsuit warned that insufficient prototype testing could put passengers in danger

• The vessel is a submersible not a submarine, and there is a key difference