In 2000, Dr. Michael Guillen, then an ABC News correspondent, was filming from the wreck of the Titanic when his vessel became trapped in the propeller.

"When we collided with the propeller, and I started seeing those big chunks of metal raining down on us … the first reaction I had was, 'This can't be happening,'" he recalled to ABC News on Tuesday.

"We got caught by this underwater turn and just drove us right into the blades," Guillen said.

"This voice came into my head and said, you know, 'This is how it's going to end for you,'" he recalled. "I'll never forget those words."

"I'm very aware of what these poor souls on board the ship the Titan are experiencing," he said. "I am just heartbroken about it."

At the request of French President Emmanuel Macron, a French ship named Atalante is diverting to the area of the missing submissive and should reach the area Wednesday night.

The ship has an exploration robot that can dive up to 4,000 meters, according to a spokesman for France's Ifremer Institute.

Per Wimmer, an explorer and global financier, told ABC News he was "shocked" and "horrified" to hear of the missing submersible.

"It felt very personal. … I've been wanting to go down to the Titanic for more than a decade," Wimmer said.

He estimated that there's been between 150 and 200 missions to the Titanic.

There "are very, very few submersibles in the whole world that can go down to the depth of Titanic, which sits at 3,800 meters, or about 12,000 feet," Wimmer said. "It's very deep -- most submersibles can go down to about 1,000 meters."

"It's a race against time, because there's only 96 hours of oxygen on board. And after that, if you haven't reached the surface, you starve of oxygen," he said.

"Our best hope at the moment is that the safety mechanism will be activated," he said. "It is supposed to be able to float to the surface, little by little, and then they can open the hatch and hopefully get out again. That is the only hope we have, because you do not have enough time to get another submersible that can go that deep."

-ABC News' Dom Proto

Renowned explorers and a father-son duo were among the five people aboard a submersible that disappeared in the Atlantic Ocean on Sunday while touring the Titanic wreckage, ABC News has learned.

ABC News has confirmed the identities of those aboard as: Shahzada Dawood, a Pakistani businessman, and his son Suleman Dawood; Hamish Harding, a British businessman, pilot and space tourist; Paul-Henri Nargeolet, a French diver and Titanic expert; and Stockton Rush, the CEO and founder of the submersible's Washington-based operator, OceanGate Expeditions.

-ABC News' Victoria Beale, Miles Cohen, Laryssa Demkiw and Anna Rabemanantsoa

The United States Coast Guard commander leading the search for a missing submersible off the coast of Newfoundland, Canada, said Tuesday morning that crews in multiple aircraft have flown over an area of the Atlantic Ocean "roughly about the size of Connecticut" while "looking for any signs of surfacing."

"As we continue on with the search, we're expanding our capabilities to be able to search under the water as well," Rear Adm. John Mauger, commander of the First Coast Guard District, told ABC News' Robin Roberts during an interview on "Good Morning America."

A commercial vessel with remotely operated vehicles is now on scene that will allow rescuers to search underwater, according to Mauger.

"This is a complex case," he added. "The Coast Guard doesn't have all the resources to be able to affect this kind of rescue, although this is an area that's within our search zone."

In many cases, Mauger said, the Coast Guard's role is to coordinate all of the assets and technical expertise that can be used in a search, in addition to operating aircraft or ships when necessary.

"In this particular case, we've established a unified command with the United States Navy, with the Canadian Armed Forces, with the Canadian Coast Guard and with the private operator OceanGate Expeditions to make sure that we understand what's needed and deploy all available equipment to the scene that could be used to locate this submersible, whether it's on the surface or down beneath the surface," he said.

Crews have been working "around the clock" to locate the deep-sea vessel since it lost contact with its operator on Sunday morning, according to Mauger.

In the last 24 hours, a Canadian aircraft has been dropping sonar buoys into the water that can pick up sound the submersible may be emitting. Vessels that have the capability to listen with their own sonar equipment are also on scene, according to Mauger.

"If they are making sound, that's certainly one of the ways that we're going to use to locate them," he said.