Missing Titanic sub: What rescuers are doing to find vessel with Dubai expat Hamish Harding on board

Rescue teams expanded their search underwater Tuesday as they raced against time to find a deep-diving tourist submersible that went missing near the wreck of the Titanic with five people on board and limited oxygen.

All communication was lost with the 21-foot (6.5-meter) Titan craft during a descent Sunday to the Titanic, which sits at a depth of crushing pressure more than two miles (nearly 4km) below the surface of the North Atlantic.

The submersible was carrying three fee-paying passengers -- Dubai-based British billionaire Hamish Harding, and Pakistani tycoon Shahzada Dawood and his son Suleman.

The US and Canadian Coast Guards have deployed ships and planes in an intensive search for the vessel, which is equipped with just four days of oxygen.

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Rear Admiral John Mauger, leading the search, told ABC News that rescuers had scoured an area of about 5,000 square miles (13,000sqkm).

Mauger said a P-3 plane from Canada has dropped sonar buoys in the area of the Titanic wreckage to listen for any sound from the small sub.

He added that the search, initially restricted to the ocean's surface, is now going under water as well.

France's oceanographic institute said it was sending a deep-sea underwater robot to aid efforts.

In an Instagram message posted just before the dive, Harding said a mission window had opened after days of bad weather. Among the crew he named was Paul-Henry Nargeolet, a veteran diver and expert on the Titanic wreck.

Unconfirmed reports said the fifth person on board was Stockton Rush, the CEO of OceanGate Expeditions which operates the tourist dives.

The Titan lost contact with the surface less than two hours into its descent, according to authorities.

"We are exploring and mobilising all options to bring the crew back safely. Our entire focus is on the crew members in the submersible and their families," OceanGate said in a statement.

Mike Reiss, an American television writer who visited the Titanic wreck on the same sub last year, told the BBC the experience was disorientating. The pressure at that depth as measured in atmospheres is 400 times what it is at sea level.

"The compass immediately stopped working and was just spinning around and so we had to flail around blindly at the bottom of the ocean, knowing the Titanic was somewhere there," Reiss said.

"But it's just so pitch dark that the biggest thing under the ocean was just 500 yards away and we spent 90 minutes looking for it."

He told the BBC that everyone was aware of the dangers. "You sign a waiver before you get on and it mentions death three different times on page one.

"This isn't a coach holiday or something. Things go wrong."

OceanGate Expeditions charges $250,000 for a seat on the Titan.

Harding, a 58-year-old aviator, space tourist and chairman of Action Aviation, is no stranger to daredevil antics and has three Guinness world records to his name.

A year ago, he became a space tourist through Amazon founder Jeff Bezos's Blue Origin company.

In his Instagram post, Harding said how proud he was to be part of the latest mission. "Due to the worst winter in Newfoundland in 40 years, this mission is likely to be the first and only manned mission to the Titanic in 2023," he wrote.

"The team on the sub has a couple of legendary explorers, some of which have done over 30 dives to the RMS Titanic since the 1980s including PH Nargeolet," the post added.

Shahzada and Suleman Dawood hail from one of Pakistan's richest families that runs an investment and holding company headquartered in Karachi.