Why did it happen? Could it have been prevented? BBC iPlayer has just got Implosion: The Titanic Sub Disaster about the catastrophe that shocked the world

BBC iPlayer has just added Implosion: The Titanic Sub Disaster, which is also being shown today [Tuesday, May 27] on BBC Two at 9 pm.

Two years ago, the world watched in horror as a frantic search and rescue mission was launched for the Titan submersible, which had been diving 13,000 feet beneath the Atlantic Ocean as part of a tourist expedition to explore the wreckage of the Titanic.

On board were 19-year-old Strathclyde University student Suleman Dawood and his father, Shahzada, 48. Their fellow crew members were businessman Hamish Harding, expert submariner Paul-Henri Nargeolet, and the inventor of Titan, Stockton Rush.

Just an hour and 33 minutes into the voyage, however, the submersible lost contact with its surface ship, the Polar Prince. Four frenetic days unfolded in an attempt to locate it, until the US Coast Guard confirmed the five crew had died in a catastrophic implosion.

Now, BBC iPlayer's documentary Implosion: The Titanic Sub Disaster hears from Christine Dawood, who was waiting on board the Polar Prince when she learned the fate of her husband and son.

"The moment we knew they’d found debris and there were no survivors, my daughter Alina and I went on deck," says Dawood, from Surrey. "Until that moment we’d had hope. We took some cushions with us and just sat there looking out at the ocean. We were both crying."

With unprecedented access to the Marine Board Investigation, as well as previously unseen footage from Titan’s previous dives, the documentary reveals how the submersible had equipment issues leading up to the disaster.

Not only that, but Stockton Rush, the founder and CEO of OceanGate, which built, owned and operated Titan, had openly boasted about breaking rules. So could the implosion have been prevented?

"In hindsight, would I have wanted them not to go? Absolutely, but I can’t really say I’d have denied them an opportunity like that," says Dawood.

"Sometimes I still don’t believe it. The possibility of Titan imploding never crossed our minds.

"To lose a husband is terrible, but no parent should have to grieve for their child. It’s unnatural. All of a sudden your purpose, your identity, is ripped away from you."

Implosion: The Titanic Sub Disaster is on BBC iPlayer now.