Titanic tourist submersible: My trip aboard the sub

A journalist who travelled aboard a tourist submersible that has now gone missing during a Titanic wreck dive says it would be impossible for those inside it now to escape without help.

He said it would be challenging for rescuers to find the vessel in time.

Speaking to the BBC on Monday, Pogue explained that passengers were sealed inside the main capsule by several bolts that were applied from the outside and had to be removed by an external crew.

He told The Context programme that the craft, which is believed to be OceanGate's Titan submersible, had seven different functions to allow it to resurface and that it was "really concerning" none of these had so far worked.

However, Pogue said the vessel's resurfacing capabilities would be irrelevant if the sub became trapped or sprang a leak.

"There's no backup, there's no escape pod," he said. "It's get to the surface or die".

Rescuers are racing against time to locate the missing sub, which normally carries around four days-worth of oxygen for a crew of five, according to the US Coast Guard.

Government agencies, the US and Canadian navies and commercial deep-sea firms are helping the rescue operation, officials said.

Titanic's wreck lies some 435 miles (700km) south of St John's, Newfoundland, though the rescue mission is being run from Boston, Massachusetts.

Further complicating the recovery mission is the fact that GPS does not work underwater, nor does radio - meaning there is currently "no way" to communicate with the vessel.

"When the support ship is directly over the sub, they can send short text messages back and forth. Clearly those are no longer getting a response," Pogue said, adding that Titan had got lost for about three hours during the expedition he was on last year.

He described being initially hesitant about going aboard the sub at all because some of the components appeared "off the shelf, sort of improvised".

"You steer this sub with an Xbox game controller, some of the ballast is abandoned construction pipes."

Pogue said he had been reassured by Titan's inventor and OceanGate's CEO, Stockton Rush, that the carbon-fibre main capsule had been co-designed with Nasa and the University of Washington and was "rock solid".

In a statement on Monday, OceanGate said its "entire focus [was] on the crewmembers in the submersible and their families".

The company added that it was "deeply thankful" for the "extensive assistance" it had been given from government agencies and companies involved in deep sea operations.

The Titanic, which was the largest ship of its time, hit an iceberg on its maiden voyage from Southampton to New York in 1912. Of the 2,200 passengers and crew onboard, more than 1,500 died.

Its wreckage has been extensively explored since it was discovered in 1985.