What was supposed to be a 10-hour journey to the Titanic shipwreck ended in tragedy, with all five passengers on the missing submersible killed in a catastrophic implosion, writes CNN.

“Their deaths were confirmed Thursday, concluding a week-long search for survivors that was closely watched around the world.

An underwater implosion refers to the sudden inward collapse of the vessel, which would have been under immense pressure at the depths it was diving toward.

It’s unclear where or how deep the Titan was when the implosion occurred, but the Titanic wreck sits nearly 13,000 feet (or almost 4,000 meters) below sea level. The submersible was about 1 hour and 45 minutes into the roughly two-hour descent when it lost contact.

At the depth the Titanic rests, there is around 5,600 pounds per square inch of pressure – several hundred times the pressure we experience on the surface, according to Rick Murcar, the international training director at the National Association of Cave Diving.

A catastrophic implosion is “incredibly quick”, taking place within just a fraction of a millisecond, said Aileen Maria Marty, a former Naval officer and professor at Florida International University.

“The entire thing would have collapsed before the individuals inside would even realize that there was a problem”, she told CNN. “Ultimately, among the many ways in which we can pass, that’s painless”.

Experts say it is unlikely any bodies will be recovered.