Questions are mounting about how a Titanic tourist submersible carrying five people could have vanished, as time runs out to find it. Neetu Garcha speaks with Titanic expert and author Tim Maltin about the kind of conditions someone would face at the depths of the North Atlantic, what to make of the timeline of the vessel's disappearance, and a New York Times report claiming OceanGate Expeditions was warned in 2018 of potential "catastrophic" problems of such endeavours.