The four day-long search for the missing Titan submersible has come to a tragic end, with all of the five people on board the vessel believed to be dead, the US Coast Guard (USCG) said on Thursday.

The Coast Guard earlier announced that rescuers had uncovered a "debris field" within the search area where teams were searching for the missing Titan submersible that was traveling to the wreck of the Titanic.

A remote-controlled underwater search vehicle (ROV) found pieces of debris on the ocean floor that belonged to the Titan submersible, approximately 488 meters (1,600 feet) away from the bow of the sunken Titanic on the ocean floor.

The debris found indicates "a catastrophic implosion of the vessel," USCG Rear Admiral John Mauger said at a press conference on Thursday in the US city of Boston.

How did the submersible implode?

There is no definitive proof of what happened, but experts assume the implosion happened on Sunday — the first day of the dive.

The Titan was made of carbon fiber and titanium, materials thought to withstand the pressure at depths of up to 4,000 meters. The craft's hull was designed to protect the crew from the water pressure.

The water pressure 3,800 meters down at the site of the Titanic wreck is roughly 400 atmospheres (6,000 PSI) — about the same as having 35 elephants on your shoulders.

"Any deep divers know how unforgiving the abyssal plain is: going undersea is as, if not more, challenging than going into space from an engineering perspective," said Eric Fusil, director of the Shipbuilding Hub for Integrated Engineering at the University of Adelaide, Australia.

Experts have questioned the use of titanium and carbon fiber for deep diving, as they have different properties.

Titanium is elastic and can adapt to ranges of stresses and pressures without permanent strain on the material. Carbon fiber on the other hand is stiffer and non-elastic, often prone to cracking.

Eric Fusil, writing on Friday in The Conversation, speculates the differences in the materials could have created a defect in the hull, triggering “an instantaneous implosion due to the underwater pressure.”

“Within less than one second, the vessel — being pushed down on by the weight of a 3,800m column of water — would have immediately crumpled in from all sides,” Fusil wrote.

The implosion would have killed everyone within less than 20 milliseconds, a speed faster than the brain can even process the information.

Exploring the ocean depths 

This is not the first deep-sea expedition in which tourists have explored the deepest recesses of the ocean.

In 2012, Canadian filmmaker James Cameron set off on a deep-sea expedition to the deepest point on Earth in the Mariana Trench aboard the submersible Deepsea Challenger. He collected data and footage at depths of around 10,900 meters.

His deep-sea voyage came exactly 100 years after the Titanic sank to the ocean floor. Cameron has also visited the Titanic wreck numerous times in earlier deep-sea voyages.

In 2019, American explorer and private investor Victor Vescovo set a world record in the Mariana Trench, where he reached 10,928 meters — 16 meters deeper than the previous record set by Don Walsh and Jacques Piccard in 1960.

Together with billionaire Hamish Harding (passenger on the Titan believed to be deceased), Vescovo also set a world record for the longest time spent in the deepest part of the ocean on a single dive when they spent four hours and 15 minutes traversing the Mariana Trench in 2021.

On board the Titan was also the former French Navy Captain Paul-Henri Nargeolet, who has often dived to the Titanic wreck. A few years ago, in an interview with the Irish Examiner newspaper, he said, "In deep water, you're dead before you can realize what's happening."

The 77-year-old had been head of the research program at RMS Titanic/Phoenix International, which owns the salvage rights to the wreck, since 2007.

Edited by: Helen Whittle