As an international search-and-rescue mission to find the missing OceanGate Titan sub took place this week involving militaries and commercial assets, authorities and subject matter experts have repeatedly referred to remotely operated vehicles (ROVs).

"The acronym Remotely Operated Vehicle, the original use is underwater robot that is completely operated by a human operator," explained Dr. Brendan Englot, an expert on undersea robotics at the Stevens Institute of Technology in New Jersey. "The analogy might be like the kind of person who operates a piece of construction equipment or a shipping crane – a very high-stakes, high-precision remote operation task, [and] highly trained operator."

Advanced ROVs are "kind of like a drone," he said, except they are controlled manually.

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Experts say that ROVs are the best bet to attempt a rescue of the OceanGate Titan submersible, which vanished Sunday during a dive toward the Titanic shipwreck in the Atlantic Ocean, about 900 miles east of Cape Cod, Massachusetts.

On board are OceanGate CEO Stockton Rush; British businessman turned adventurer Hamish Harding, father-and-son Shahzada and Suleman Dawood, who are members of one of Pakistan’s wealthiest families; and Paul-Henry Nargeolet, a former French navy officer and leading Titanic expert.

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While ROVs vary in design and capability, they can generally travel much deeper than manned vessels, Englot said.

"Those kind of vehicles usually have robotic arms that are capable of carrying a payload, grasping an object, grabbing and turning a knob or a valve or something like that," he added.

As of Thursday morning, several with the ability to reach the ocean floor had been deployed in the Atlantic as the Titan’s estimated initial supply of 96 hours of oxygen dwindled – including the Victor 6000, which descended from the French L'Atalante research vessel to the ocean floor.

A Canadian vessel, the Horizon Arctic, also deployed its ROV Thursday morning, according to the U.S. Coast Guard, which is coordinating a unified command of the effort.

"They have a very large capable vehicle deployed in the water," Englot told Fox News Digital. "It will be tethered with the operator sitting at the surface driving it."

If rescuers discover that the Titan sub is intact but trapped near the seafloor, an ROV would be the likely asset to effect a retrieval, according to Dr. Jeff Karson, an experienced deep-sea explorer and professor emeritus of earth and environmental sciences at Syracuse University.

While ROVs have limited visibility themselves due to underwater conditions, they are guided to locations of interest by sonar from surface vessels and buoys dropped from the sky.

The mission may be as simple as helping the sub detach weights that somehow got stuck, allowing it to use its natural buoyancy to float back to the surface, he said.

"You should be able to do that with a simple turn of a wrench or something down there, but it hasn't happened," he said. "So that suggests something, either they're hung up in a very complicated way or there was some massive failure down there long ago, a couple of days ago, and there's nothing to be done."

Fox News' Haley Chi-Sing and Julia Bonavita contributed to this report.